feat(gastown): add proactive idle-container stop in TownDO alarm

[jrf0110]

Summary

Add proactive idle-container stop to the TownDO alarm handler. When a town has no active work and the mayor has been idle for >5 minutes, the alarm now calls container.stop() to force a graceful SIGTERM drain instead of waiting for Cloudflare's port-idle timer (which gets reset by PTY WebSocket keep-alives from dashboard terminals). This targets the root cause of 300+ active containers for ~100 active users.

• New stopContainerIfIdle() method with dependency-injected logic in town/container-idle-stop.ts for testability
• Wired into alarm handler just before the re-arm block
• Emits container.idle_stop event with reason (no_active_work / mayor_idle_Xm) for observability
• 2-minute throttle prevents thrash; failed stops are retried next tick (throttle not set on failure)
• 13 unit tests covering all branches including edge cases (null mayor, null last_activity_at, getState failure, stop failure, throttle)

Verification

• Created a town with a running container, confirmed the alarm fires stopContainerIfIdle() after the idle threshold
• All 13 unit tests pass

Visual Changes

N/A

Reviewer Notes

• The implementation delegates to a standalone sub-module (container-idle-stop.ts) with injected dependencies, making it fully testable without DO infrastructure
• getState() is used (control-plane RPC) instead of fetch() or warmUp() to avoid waking a sleeping container
• The ! non-null assertion on mayor.last_activity_at was removed in favor of != null guards, consistent with the project coding style

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• services/gastown/src/dos/Town.do.ts
• services/gastown/src/dos/town/container-idle-stop.ts
• services/gastown/src/dos/town/container-idle-stop.test.ts

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_{Reviewed by gpt-5.5-2026-04-23 · 318,978 tokens}