fix(autopilot): cancel the between-cycle sleep on SIGTERM/SIGINT (#204)

[sharziki]

Summary

Fixes #204 — under systemd, gbrain autopilot gets SIGKILL'd before the drain path runs because the between-cycle setTimeout is not cancelable.

Root cause

The autopilot loop waited for the next cycle with:

await new Promise(r => setTimeout(r, interval * 1000));

The SIGTERM/SIGINT handler flips `stopping = true`, but the `while (!stopping)` guard only re-evaluates after the current sleep resolves. With adaptive intervals scaling up to 600s on a healthy brain, systemd's default `TimeoutStopSec=90` loses the race, SIGKILL preempts the shutdown path, and the lockfile at `~/.gbrain/autopilot.lock` is left stale — the next invocation then has to either wait out the 10-minute staleness check or be cleaned up by hand.

Fix

• Extract `sleepCancelable(ms, signal)` helper — resolves after `ms` or rejects with an `AbortError` the moment the signal aborts.
• Add a module-scope `AbortController` (`cycleAbort`) in `runAutopilot`.
• Abort `cycleAbort` from the `shutdown` handler so the loop wakes up immediately.
• Catch only the abort-path rejection; let any other unexpected rejection propagate so we don't hide real bugs.

No behavior change in the worker-drain path (35s `Promise.race` stays intentional — it bounds how long we wait for the Minions worker to drain).

Test plan

• New tests in `test/autopilot-sleep-cancelable.test.ts` covering:
  • resolves normally when the signal never aborts
  • rejects synchronously when the signal is already aborted
  • rejects with AbortError when the signal aborts mid-sleep
• Existing tests still pass:
  • `bun test test/autopilot-sleep-cancelable.test.ts test/autopilot-install.test.ts test/autopilot-resolve-cli.test.ts` → 10/10 pass.

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