2pc: drop the pull-through registry cache — single host makes it marginal; keep PC1 prune-policy only

Operator (2026-05-29): on one host Docker's local image store already IS the cache; the churn was
over-pruning, not a missing cache. So 2pc = conservative prune policy + confirm local-store retention
+ daemon auth (PC1-3). Registry pull-through cache deferred to IDEAS with a concrete revisit
condition (multi-node, or measured cold-deploy bottleneck on recreate-surviving storage).

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

cc-ci-plan/IDEAS.md

@@ -30,6 +30,17 @@ item into the project `BACKLOG.md` as `[idea]` if/when it becomes relevant.
     brittle, or if maintainers strongly prefer normal coop-cloud workflow over the Nix layer — weigh
     that against how much we value full reproducibility (D8) + hands-off auto-updates. *Added:* 2026-05-29.
 
+- [ ] **Docker Hub pull-through registry cache (`registry:2` proxy) — deferred; single-host makes it marginal.**
+  Considered as a Phase-2pc perf win, then **dropped (operator, 2026-05-29):** on a **single host**,
+  Docker's own local image store already caches pulled images (re-deploys reuse local layers), so the
+  prune-policy fix (Phase 2pc PC1) recovers ~all the benefit. A separate pull-through cache's
+  distinctive wins don't apply here — multi-node fan-out (one node), surviving prune/VM-rebuild on
+  *separate* storage (ours would be co-located, lost on recreate), cache-miss auth (daemon already
+  PAT-authenticated). **Revisit ONLY if:** (a) cc-ci goes **multi-node**, OR (b) Phase-2b measurement
+  shows **cold-cache / fresh-deploy pull time** (D8 throwaway-rebuild, fresh-canonical seeding) is a
+  real bottleneck **AND** the cache lives on **recreate-surviving storage** (Incus volume / host-b1
+  path, not the VM's ephemeral disk). Otherwise it's complexity without payoff. *Added:* 2026-05-29.
+
 - [ ] **Optional `--extra-tests` flag for heavy / operational tests (opt-in heavy suite).**
   Some recipe tests are "more than needed" for the default CI signal — state-management /
   long-running-instance / load / helper-script operational tests that don't fit the ephemeral