feat: queue concurrent local:gate runs on a machine-wide lease (#2339)

`npm run local:gate` now runs its stages under `scripts/gate-lease.mjs`, a
`proper-lockfile` lease under `os.tmpdir()` (or `$XDG_RUNTIME_DIR`), so a gate
started in a second worktree waits for the first instead of overlapping it.

Measured on v2/main @ aa56551b: a quiet gate is 257s green; two started
together had one fail at 279s (`smoke:web:chromium` found port 6298 held by
the other gate's smoke) and the survivor take 338s. Under the lease, three
gates started within seconds ran back to back (259s, then 262s, then the
third), each green with zero timeouts.

The lease grants rather than polls a threshold, so two waiters cannot
deadlock on each other; a holder that dies stops refreshing and is taken over
after 30s; a signal to the wrapper stops the whole process group, releases,
and exits 128+signal; the wait names the holder's pid and worktree and gives
up after 45 minutes. `INSPECTOR_SKIP_GATE_LEASE=1` bypasses it. A lock that
cannot be created runs the gate unleased with a warning.

`scriptChainRuns` looks through the wrapper (and only it) so the existing
vouches about what `local:gate` runs keep reading the real chain.

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>

AGENTS.md

@@ -476,6 +476,7 @@ free to run a full `local:gate`). Raising a budget nobody chose hides no race.
 - **`npm run local:gate` is the mandatory pre-push command.** It runs **every check** `.github/workflows/main.yml` runs, plus two local-only steps, so a green run here is the strongest predictor of a green CI this repo has: every check CI applies has already passed on your machine. It is not a proof — CI runs on a different OS and, since #2341, runs each client's suite bare where the gate runs it only instrumented — so the one residual is a test that passes *only* when slowed down, which is a race (#1596) to fix, never headroom to keep. Expect several minutes.
 - **The gate runs each client's test suite once, instrumented; CI runs it twice (#2341).** CI's `build` job runs the bare `test` inside `validate` and its parallel `coverage` job runs `test:coverage`, which costs CI no wall clock. Serially in one process the bare pass was ~80s of a ~370s gate, re-running exactly the files the coverage pass runs a few minutes later — so the gate calls **`local:validate`**, which is `validate` minus each client's `test` leg (every client's `validate` is `check && test`, and `local:validate` runs the `check` half). "Every check" is therefore a claim about checks, not invocations, and it holds because `@vitest/coverage-v8` collects coverage from V8's own profiler (`Profiler.takePreciseCoverage`) and rewrites no source: a test file sees identical code either way, and the instrumented run is only *slower*, which makes it the stricter of the two for the failure class this repo actually sees (a correct test cut off under load). A test that passed only *because* it ran slower would be a race — #1596's class, a defect wherever it surfaces — and CI's bare pass still runs it. **`npm run validate` is unchanged**, because CI runs it directly and it is the inner-loop check; `verify:*` guards that ask "is this reachable from `validate`" are unaffected. `local:validate` lives in the `local:` namespace so the workflow guard keeps it out of CI by construction.
 - **`npm run validate` is the fast inner-loop check and is NOT an acceptable substitute.** It runs `test`, not `test:coverage`, so it does **zero** coverage gating, no smokes, and no Storybook tests. Skipping the gate is how a push passes every fast local check and still fails CI.
+- **Concurrent gates queue; they do not overlap.** `local:gate` takes a machine-wide lease (`scripts/gate-lease.mjs`, #2339) so a gate started in a second worktree waits for the first rather than running alongside it. Overlap is not merely slow — the web smokes bind fixed ports, so two gates reaching the same smoke together go red on a diff that cannot have caused it (measured: one of two concurrent gates failed at 279s on port 6298 while a quiet gate passed in 257s). The wait names the holder and its worktree; a holder that dies releases within 30s; `INSPECTOR_SKIP_GATE_LEASE=1` bypasses it, which is for a measurement that _needs_ contention, never for getting a result sooner — the queued run finishes sooner anyway.
 - There is deliberately **no `npm run ci`** — that name collided with the `npm ci` built-in, which clean-installs from the lockfile and does not run this script.
 - What each stage covers, and why two of them are local-only, is [`docs/quality-gate.md`](./docs/quality-gate.md); how to diagnose a failing stage is the `pre-push-gate` skill.