feat(2pc): PC1 conservative prune — drop autoPrune --all, add gated surgical docker-prune

Removes virtualisation.docker.autoPrune (daily `docker system prune --all` evicted in-use base
images → cold re-pull → Hub rate-limit churn, JOURNAL-2). Adds modules/docker-prune.nix: daily
timer + oneshot that prunes only dangling+until=24h, gated on disk pressure (>=80%) AND no run-app
live AND no swarm service converging; never --all, never --volumes. Teardown unchanged (never
removes images). Registry pull-through cache dropped per operator scope correction.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

machine-docs/BACKLOG-2pc.md

@@ -0,0 +1,26 @@
+# BACKLOG — Phase 2pc (sane image-prune policy)
+
+SSOT: `/srv/cc-ci/cc-ci-plan/plan-phase2pc-image-cache.md`.
+Scope (post operator correction 2026-05-29): **PC1 prune policy + confirm local-store
+retention/auth ONLY.** The registry:2 pull-through cache is **dropped** (deferred to IDEAS /
+Phase 2b — revisit only if multi-node OR a measured cold-deploy bottleneck on recreate-surviving
+storage).
+
+## Build backlog
+
+- [ ] **PC1 — Conservative prune policy.** Remove `virtualisation.docker.autoPrune` (`--all` evicts
+      in-use base images → forced cold re-pull → rate-limit). Replace with a surgical, gated prune:
+      dangling + `until=24h` only, NEVER `--all`/`--volumes`; gated on (a) genuine disk pressure
+      (`/` ≥ 80%), (b) no run-app stack live, (c) no swarm service converging (mid-pull). Teardown
+      already removes only services/volumes/secrets/.env — NOT images (verified) — keep it that way.
+- [ ] **PC2 — Confirm local cache retained + authenticated.** Daemon stays PAT-authenticated
+      (`docker info` Username=nptest2, sops `dockerhub_auth` → `/root/.docker/config.json`); local
+      image store `/var/lib/docker` persists across runs/teardowns/reboots. No code change expected —
+      confirm + document.
+- [ ] **PC3 — Verify + document.** Deploy → teardown → redeploy reuses local layers (no
+      re-download); disk bounded without `-af`. Update `docs/runbook.md` + `docs/` prune note;
+      record the policy + the dropped-registry-cache deviation in `DECISIONS.md`.
+
+## Adversary findings
+
+(Adversary owns this section.)

machine-docs/JOURNAL-2pc.md

@@ -0,0 +1,47 @@
+# JOURNAL — Phase 2pc (sane image-prune policy)
+
+Append-only reasoning log. Facts/verification for the Adversary live in STATUS-2pc.md.
+
+## 2026-05-29 — Orientation + scope correction
+
+Read SSOT `plan-phase2pc-image-cache.md` + plan.md §6.1/§7/§9. Operator issued a **scope
+correction** mid-orientation: **drop the registry:2 pull-through cache.** Rationale (operator):
+single host → Docker's own local image store already IS the cache; re-deploys reuse local layers
+with no re-download; the daemon is PAT-authenticated so residual manifest checks sit under 200/6h.
+The churn was caused by **over-pruning** (`docker image prune -af` wiping the store), not a missing
+cache. A separate registry only pays off multi-node / separate-survivable storage, which we are not.
+**I had not yet written any registry code** (still orienting) → nothing to revert.
+
+Phase 2pc is now **PC1 (prune policy) + PC2/PC3 (confirm + verify local-store retention/auth).**
+
+### Findings from orientation (why the fix is one module)
+
+- The ONLY automated image pruner in the whole repo is
+  `virtualisation.docker.autoPrune = { flags = ["--all" "--filter" "until=24h"]; }` in
+  `nix/modules/swarm.nix`. NixOS renders this as `docker system prune --force --all --filter until=24h`
+  daily. `--all` removes every image **not used by a running container** — between runs there are no
+  test apps running, so it evicts the cached recipe base images → cold re-pull on the next run. That
+  is exactly the prune→re-pull→rate-limit churn documented in JOURNAL-2 (lines 507/542/690-693).
+- `runner/harness/lifecycle.py::teardown_app` removes services (abra undeploy / `docker stack rm`),
+  volumes, secrets, and the `.env` — and **no images** (`grep` for `rmi`/`image rm`/`image prune` in
+  `runner/` + `tests/conftest.py` is empty). So PC1's "teardown must NOT remove images" already holds.
+- `janitor`, `warm_reconcile.py`, `nightly-sweep.nix`, `drone*.nix`, `.drone.yml` — none prune images.
+- Daemon is already PAT-authenticated: `docker info` → `Username: nptest2`; sops `dockerhub_auth`
+  (base64 `nptest2:<PAT>`) → `sops.templates."docker-config.json"` → `/root/.docker/config.json`
+  (`nix/modules/secrets.nix`). PC2 needs no change — confirm + document.
+- Disk on cc-ci: `/` is 64G, 19G used, **43G free (31%)** — bounded; aggressive `--all` is
+  unnecessary, which is the whole premise.
+
+### PC1 design
+
+Replace `autoPrune` with a dedicated `nix/modules/docker-prune.nix`: a daily `systemd.timer` +
+oneshot `systemd.service` running a surgical, **triple-gated** prune:
+1. **Disk-pressure gate** — do nothing unless `/` usage ≥ 80% (Docker's local store IS our cache;
+   keep it warm; reclaim only under genuine pressure).
+2. **No-run gate** — skip if any run-app stack (`<=4char>-<6hex>_ci_commoninternet_net_*`) is live
+   (mid-pull layers can look prunable; "never prune mid-run").
+3. **No-converge gate** — skip if any swarm service has unmet replicas (a deploy/pull in flight,
+   incl. infra warm redeploys).
+When all gates pass: `docker {container,image,builder} prune -f --filter until=24h` — dangling +
+age-gated only. NEVER `--all` (keeps tagged base/in-use images), NEVER `--volumes` (warm canonical
+data, per swarm.nix's existing comment).

machine-docs/STATUS-2pc.md

@@ -0,0 +1,22 @@
+# STATUS — Phase 2pc (sane image-prune policy)
+
+**SSOT:** `/srv/cc-ci/cc-ci-plan/plan-phase2pc-image-cache.md`
+**Scope (operator correction 2026-05-29):** PC1 conservative prune + PC2/PC3 confirm-and-verify
+local-store retention/auth. **Registry pull-through cache DROPPED** (deferred to IDEAS / Phase 2b).
+
+## Phase: PC1 implemented, deploy+verify in flight (NOT yet claimed)
+
+In flight: build the new prune module onto cc-ci via `nixos-rebuild switch`, then run the
+deploy→teardown→redeploy layer-reuse proof. Gate will be CLAIMED once verified on the real host.
+
+## What changed (the diff)
+
+- `nix/modules/swarm.nix` — removed `virtualisation.docker.autoPrune` (it ran
+  `docker system prune --force --all --filter until=24h` daily; `--all` evicts every image not used
+  by a *running* container → wiped cached recipe base images → cold re-pull → Hub rate-limit churn).
+- `nix/modules/docker-prune.nix` (NEW) — daily `systemd.timer` + oneshot `systemd.service`
+  `docker-prune` running a surgical, triple-gated prune. Imported in `nix/hosts/cc-ci/configuration.nix`.
+- Teardown (`runner/harness/lifecycle.py::teardown_app`) UNCHANGED — already removes only
+  services/volumes/secrets/.env, never images (PC1 teardown requirement already held).
+
+(Verification context — WHAT/HOW/EXPECTED/WHERE — will be filled in here at gate-claim time.)

nix/hosts/cc-ci/configuration.nix

@@ -8,6 +8,7 @@
     ../../modules/packages.nix
     ../../modules/secrets.nix
     ../../modules/swarm.nix
+    ../../modules/docker-prune.nix
     ../../modules/abra.nix
     ../../modules/proxy.nix
     ../../modules/drone.nix

nix/modules/docker-prune.nix

@@ -0,0 +1,75 @@
+# Conservative, surgical Docker prune (Phase 2pc / PC1).
+#
+# REPLACES `virtualisation.docker.autoPrune` (which ran `docker system prune --force --all
+# --filter until=24h` daily). The `--all` removed every image NOT used by a *running* container —
+# between CI runs no test apps run, so it evicted the cached recipe base images and forced a cold
+# re-pull on the next run → the prune->re-pull->Docker-Hub-rate-limit churn documented in JOURNAL-2.
+#
+# On this SINGLE host, Docker's own local image store IS the cache (re-deploys reuse local layers,
+# no re-download; the daemon is PAT-authenticated). So we keep that store warm and only reclaim disk
+# under GENUINE pressure, and even then SURGICALLY:
+#   - dangling images + stopped containers + dangling build cache, age-gated (until=24h) — NEVER
+#     `--all` (would evict tagged base/in-use images), NEVER `--volumes` (warm canonical data — see
+#     swarm.nix's existing comment; warm volumes are reaped only by the warm reconcilers).
+# and only when nothing is in flight:
+#   - skip if any run-app stack is live (mid-pull layers can look prunable — "never prune mid-run");
+#   - skip if any swarm service has unmet replicas (a deploy/pull is converging, incl. warm redeploys).
+{ pkgs, ... }:
+let
+  # `/` usage % at/above which a surgical prune is permitted. Below this: keep the cache, no-op.
+  threshold = 80;
+  prune = pkgs.writeShellApplication {
+    name = "cc-ci-docker-prune";
+    runtimeInputs = with pkgs; [ docker coreutils gnugrep gawk ];
+    text = ''
+      THRESH=${toString threshold}
+      used="$(df --output=pcent / | tail -1 | tr -dc '0-9')"
+      : "''${used:=0}"
+      if [ "$used" -lt "$THRESH" ]; then
+        echo "docker-prune: / at ''${used}% (< ''${THRESH}%) — keeping local image cache, nothing to do"
+        exit 0
+      fi
+      # NEVER prune mid-run: a live run-app stack means a deploy/test is in flight (mid-pull layers
+      # can look prunable). Run-app services: <=4char>-<6hex>_ci_commoninternet_net_* (lifecycle.py).
+      if docker service ls --format '{{.Name}}' \
+           | grep -qE '^[a-z0-9]{1,4}-[0-9a-f]{6}_ci_commoninternet_net_'; then
+        echo "docker-prune: a run-app stack is live — skipping (never prune mid-run)"
+        exit 0
+      fi
+      # NEVER prune while ANY swarm service is converging (unmet replicas => a pull/deploy in flight,
+      # including infra warm redeploys). Replicas field is "running/desired" e.g. 1/1.
+      converging="$(docker service ls --format '{{.Replicas}}' \
+                      | awk -F/ '{ if (($1+0) != ($2+0)) c++ } END { print c+0 }')"
+      if [ "$converging" -gt 0 ]; then
+        echo "docker-prune: $converging service(s) converging (deploy/pull in flight) — skipping"
+        exit 0
+      fi
+      echo "docker-prune: / at ''${used}% (>= ''${THRESH}%) — surgical prune (dangling + until=24h; NEVER --all/--volumes)"
+      docker container prune -f --filter until=24h || true
+      docker image prune     -f --filter until=24h || true
+      docker builder prune   -f --filter until=24h || true
+      df -h /
+    '';
+  };
+in
+{
+  systemd.services.docker-prune = {
+    description = "Surgical disk-pressure-gated Docker prune (dangling+old only; never --all/--volumes; never mid-run)";
+    after = [ "docker.service" ];
+    requires = [ "docker.service" ];
+    path = [ pkgs.docker ];
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = "${prune}/bin/cc-ci-docker-prune";
+    };
+  };
+
+  systemd.timers.docker-prune = {
+    description = "Daily timer for the surgical Docker prune";
+    wantedBy = [ "timers.target" ];
+    timerConfig = {
+      OnCalendar = "daily";
+      Persistent = true;
+    };
+  };
+}

nix/modules/swarm.nix

@@ -5,18 +5,14 @@
 {
   virtualisation.docker = {
     enable = true;
-    # Reclaim disk from churning per-run images (cc-ci root is ~28 GiB). Prune images/containers/
+    # Image pruning is handled by modules/docker-prune.nix (Phase 2pc / PC1), NOT by
-    # networks/build-cache older than 24h — but NEVER volumes:
+    # `virtualisation.docker.autoPrune`. The old autoPrune ran `docker system prune --all` daily;
-    #  (1) `--volumes` is incompatible with `--filter until=` (docker errors → the unit failed daily,
+    # `--all` evicts every image not used by a *running* container — between runs that wiped the
-    #      degrading the system and never actually pruning — that's why disk crept to 96%); and
+    # cached recipe base images and forced a cold re-pull → the Docker-Hub-rate-limit churn in
-    #  (2) Phase 2w keeps DATA-WARM canonical volumes that are UNDEPLOYED (no container), so
+    # JOURNAL-2. The replacement keeps Docker's local store warm (it IS our cache on this single
-    #      `prune --volumes` would DELETE the warm known-good data. Warm volumes are pruned
+    # host) and prunes only dangling+old layers, gated on genuine disk pressure and nothing in
-    #      deliberately by the warm reconcilers (WC8), never by this blanket sweep.
+    # flight. NEVER --volumes either: Phase-2w keeps DATA-WARM undeployed canonical volumes, reaped
-    autoPrune = {
+    # only by the warm reconcilers. autoPrune left OFF (the default) on purpose.
-      enable = true;
-      dates = "daily";
-      flags = [ "--all" "--filter" "until=24h" ];
-    };
   };
 
   environment.systemPackages = [ pkgs.docker ];