fix(gastown): guard eviction resets with heartbeat

[jrf0110]

Summary

• Guard agentCompleted container-eviction bead resets with the same 90s heartbeat freshness window used by the reconciler.
• Share the heartbeat staleness constant/helper so live polecats keep their in-progress bead assignment while still unhooking the exited agent process.
• Add regression coverage for fresh, stale, and missing heartbeats plus the Town DO fresh-heartbeat eviction path.

Verification

Not manually tested; non-visual Durable Object state-machine change.

Visual Changes

N/A

Reviewer Notes

Local automated checks were attempted after PR creation per issue instructions, but this shell has no pnpm, Node, Corepack, or Nix available on PATH, so pnpm format, pnpm typecheck, pnpm lint, and pnpm test could not start. The branch was pushed with --no-verify as requested.

Fixes #3538

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Clean refactor that correctly guards container-eviction resets with the heartbeat freshness window; the extracted staleness.ts module, updated agentCompleted logic, and test coverage are all consistent and correct.

Files Reviewed (5 files)

• services/gastown/src/dos/town/staleness.ts — new module, clean extraction
• services/gastown/src/dos/town/staleness.test.ts — unit tests for heartbeatStale
• services/gastown/src/dos/town/reconciler.ts — constants replaced with shared HEARTBEAT_STALE_MS
• services/gastown/src/dos/town/review-queue.ts — eviction guard with fresh-heartbeat check
• services/gastown/test/integration/reconciler.test.ts — integration coverage for fresh-heartbeat eviction path

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Behaviour verified:

• Fresh heartbeat → bead stays in_progress, agent is unhooked and set idle; the stale assignee_agent_bead_id on the bead remains intact so the Reconciler Rule 3 (5 min STALE_IN_PROGRESS_TIMEOUT_MS) naturally cleans it up if no agent reconnects — this is the intended design.
• Stale heartbeat → existing reset-to-open path fires as before.
• reconciler.ts lines 725 and 904 now reference the shared constant instead of duplicate 90_000 literals.

Fix these issues in Kilo Cloud

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_{Reviewed by claude-sonnet-4.6 · 802,954 tokens}

_{Review guidance: REVIEW.md from base branch main}