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