feat: add StaleExpirationMap utility for TTL-based cache expiration

- Automatically removes entries not accessed within TTL
- Periodic cleanup with configurable interval
- Optional onExpire callback for cleanup notifications
- Refresh TTL on get (configurable)
- Touch, peek, getAge, getRemainingTtl methods
- Full iteration support
- Implements Disposable interface

Useful for caching ephemeral data like pending tool calls, subagent activity.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/utils/index.ts

@@ -9,3 +9,4 @@
 export { BufferAccumulator } from './buffer-accumulator.js';
 export { LRUMap, type LRUMapOptions } from './lru-map.js';
 export { CleanupManager, type TimerOptions } from './cleanup-manager.js';
+export { StaleExpirationMap, type StaleExpirationMapOptions } from './stale-expiration-map.js';