feat(skills): add debounced watcher foundation for phase 2

src/skills/index.ts

@@ -2,3 +2,5 @@ export type { SkillTier, SkillRequirements, SkillManifest, Skill } from './types
 export { checkRequirements, loadSkill, discoverSkills, loadAllSkills } from './loader.js';
 export { SkillRegistry } from './registry.js';
 export { SkillInstaller } from './installer.js';
+export { SkillsWatcher } from './watcher.js';
+export type { SkillsWatcherConfig, SkillsWatcherEvent } from './watcher.js';