perf: implement phase 1-3 performance optimizations

Add implementation plans and code structure analysis for a 3-phase
performance optimization effort. Refactor core modules to reduce
timer overhead, consolidate regex usage, extract exec timeout config,
add debouncer utility, and streamline server/schema validation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/image-watcher.test.ts

@@ -121,6 +121,7 @@ describe('ImageWatcher', () => {
 
     it('should be safe to call for non-watched session', () => {
       expect(() => watcher.unwatchSession('nonexistent')).not.toThrow();
+      expect(watcher.getWatchedSessions()).toHaveLength(0);
     });
 
     it('should clear pending debounce timers for the session', () => {