Recover stale Slack active turns

[zzj3720]

Summary

• Add a configurable SLACK_STALE_ACTIVE_TURN_AFTER_MS watchdog for long-silent active turns.
• Reset stale inProgress/unknown turns by interrupting the runtime when possible, moving the inflight batch back to pending, and clearing the broker activeTurnId.
• Add the approved proposal doc plus config/reconciler regression tests.

Validation

• pnpm exec vitest run test/slack-turn-reconciler.test.ts test/config.test.ts
• pnpm build
• pnpm test
• Manual smoke: ran a pnpm exec tsx snippet that simulated a stale in-progress turn and verified the reconciler called interrupt, reset-to-pending, and cleared the active turn.