fix: prevent memory leaks in background agent, tool cache, and comment checker

[code-yeongyu]

Summary

Fixes multiple memory leak issues identified in issue #165.

Changes

1. BackgroundManager Task Accumulation (manager.ts)

• Problem: Completed tasks accumulated in tasks Map forever, causing linear memory growth
• Solution:
  • Added 5-minute TTL for completed tasks (COMPLETED_TASK_RETENTION_MS)
  • Added max 50 completed tasks limit with FIFO eviction (MAX_COMPLETED_TASKS)
  • Fixed race conditions in async completion paths
  • Added timer.unref() to prevent blocking process exit
  • Clear notifications when tasks are evicted

2. Tool Input Cache (tool-input-cache.ts)

• Problem: Module-level setInterval never cleared, plus no cache size limit
• Solution:
  • Moved interval to lazy start on first use (idempotent)
  • Added process.on("exit") cleanup
  • Added max 1000 entries with FIFO eviction (MAX_CACHE_SIZE)
  • Added interval.unref() to prevent blocking process exit

3. Comment Checker (comment-checker/index.ts)

• Problem: Module-level setInterval never cleared
• Solution:
  • Moved interval to lazy start in hook factory (idempotent)
  • Added process.on("exit") cleanup
  • Added interval.unref() to prevent blocking process exit

Testing

• ✅ All existing tests pass (bun test)
• ✅ TypeScript type checks pass (bun run typecheck)

Oracle Review Summary

Oracle (GPT-5.2) reviewed these changes and confirmed:

• Fixes address real leak vectors correctly
• Race conditions are properly handled with status re-checks after async operations
• Timer unref() prevents keeping process alive just for housekeeping

Closes #165

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