chore(ci): P4 config cleanup — DRONE_RUNNER_CAPACITY is the single concurrency knob

Remove concurrency.limit from the recipe-ci pipeline (.drone.yml): it duplicated
DRONE_RUNNER_CAPACITY (nix/modules/drone-runner.nix maxTests) and the two had to be kept in
step by hand (docs/concurrency.md §8.6). maxTests comment updated to state it is the single
knob and to describe the new safety model.

.drone.yml

@@ -53,8 +53,8 @@ trigger:
   event:
     - custom
 
-concurrency:
-  limit: 2
+# NB deliberately NO `concurrency.limit` here: DRONE_RUNNER_CAPACITY (nix/modules/drone-runner.nix
+# maxTests) is the single concurrency knob (P4 — two knobs in two files drifted).
 
 steps:
   - name: ci

nix/modules/drone-runner.nix

@@ -8,18 +8,18 @@
 { pkgs, config, lib, ... }:
 let
   # MAX_TESTS (plan §4.2/§4.3 resource safety): max CI builds the exec runner runs at once. Drone
-  # queues the rest in its native pending-build queue (no custom queue). THE concurrency cap that
-  # bounds how many test apps can be live at once.
+  # queues the rest in its native pending-build queue (no custom queue). THE SINGLE concurrency
+  # knob — nothing else caps recipe-ci parallelism (the .drone.yml concurrency.limit was removed:
+  # one knob, one place). Bounds how many test apps can be live at once.
   #
   # Raised to 2 (operator request 2026-06-09) so two recipes can be tested in parallel (e.g. immich
   # and plausible under active development at once). Verified safe on the current node (Hetzner cpx22,
   # ~7.6 GiB / 4 vCPU — NOTE: smaller than the original 28 GiB this was written for): a full immich CI
   # stack measured ~1 GiB (server+ML+pg+redis) with multiple GiB free, so two concurrent recipes fit.
-  # The concurrency PRECONDITION holds: the run-start janitor is age-based (default 2h) + run-app-name
-  # scoped, so it never reaps a concurrent in-flight run (harness.lifecycle.janitor). TRADE-OFF: with
-  # capacity>1 a SIGKILL'd build (no teardown) leaves an orphan the run-start sweep can't reap
-  # immediately (it might be a live run) — bounded instead by the 2h janitor + the /upgrade-all
-  # start/end reap + sweep-orphans. Revert to "1" if OOM / disk-I/O contention is observed under load.
+  # Concurrent-run safety is the harness's job at ANY capacity (docs/concurrency.md): per-run
+  # ABRA_DIR recipe trees, per-app-domain flocks, and a flock-probe janitor that reaps a crashed
+  # build's orphan immediately (held lock = live run, never touched). Revert to "1" if OOM /
+  # disk-I/O contention is observed under load.
   maxTests = "2";
 in
 {