feat: comprehensive codebase improvements (cleanup, logic, performance)

Code Cleanups:
- Consolidated PlanTaskStatus and TddPhase types to types.ts
- Standardized node: prefix for Node.js builtin imports
- Fixed timer type to NodeJS.Timeout in respawn-controller

Logic Improvements:
- Fixed AI check race condition with UUID tracking in respawn-controller
- Added comprehensive _isStopped guards in session timer callbacks
- Made RalphTracker reset events non-reentrant via process.nextTick()
- Improved SessionManager mutex pattern reliability

Performance Optimizations:
- Fixed pendingToolCalls memory leak with TTL cleanup in subagent-watcher
- Improved LRUMap.newest() from O(n) to O(1) with _newestKey tracking

Documentation:
- Enhanced README antiflicker section with 6-layer technical details

Version: 0.1441

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

src/image-watcher.ts

@@ -8,10 +8,10 @@
  * to ensure files are fully written before emitting detection events.
  */
 
-import { EventEmitter } from 'events';
+import { EventEmitter } from 'node:events';
 import { watch, type FSWatcher } from 'chokidar';
-import { basename, extname } from 'path';
-import { statSync } from 'fs';
+import { basename, extname } from 'node:path';
+import { statSync } from 'node:fs';
 import type { ImageDetectedEvent } from './types.js';
 
 // ========== Types ==========