fix: address Copilot review round 4

- Reword the CI guarantee in AGENTS.md, docs/quality-gate.md and the
  pre-push-gate skill: a green gate means every check CI applies has
  passed here — the strongest predictor of green CI, not a proof — and
  name the residual (a test that passes only when slowed) as #1596's
  class to fix rather than headroom to keep.
- docs/quality-gate.md: cli's validate DID build, through `test`'s
  `pretest` hook; say that the build goes with the dropped `test` leg and
  `coverage:cli` builds once, later — in both places.
- Update the two source comments that still described `local:gate` as
  running `validate` (verify-skills.mjs, verify-build-gate.mjs).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PsmUAZ6aLUAFqenjJKSSRp
Signed-off-by: cliffhall <cliff@futurescale.com>

.claude/skills/pre-push-gate/SKILL.md

@@ -32,8 +32,10 @@ answer.
 
 It runs **every check** GitHub CI runs (which additionally runs `npm install`,
 and runs `coverage` as a parallel job), plus two local-only steps. So the
-direction that matters holds: **passing `local:gate` locally means CI's gates
-will pass.** The reverse does not.
+direction that matters holds: **passing `local:gate` locally means every check
+CI applies has already passed on your machine** — the strongest predictor of a
+green CI there is here, though not a proof (a different OS, and the bare test
+pass noted below). The reverse does not hold at all.
 
 One difference in *invocations*, not checks: CI runs each client's unit suite
 twice — bare inside `validate`, instrumented inside `coverage` — on two