refactor(ext/vault): extract vault-sidebar.ts with debounced search (Plan C Phase 4)

Moves the sidebar column out of vault.ts/vault-shell.ts into vault-sidebar.ts:
its markup (now incl. an empty #vault-status-slot footer for Phase 6), the
category nav rendering, nav-button wiring, and search. The search input gains
an 80ms trailing-edge debounce (P2 fix — it re-filtered on every keystroke).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/src/vault/__tests__/sidebar-glyphs.test.ts

@@ -4,7 +4,7 @@ import * as path from 'path';
 
 describe('vault sidebar glyphs', () => {
   const vaultSrc = fs.readFileSync(
-    path.resolve(__dirname, '../vault-shell.ts'),
+    path.resolve(__dirname, '../vault-sidebar.ts'),
     'utf-8',
   );