refactor: remove QMD semantic indexing — keep keyword search only

Remove the entire QMD search engine (semantic indexing, hybrid search,
vector embeddings) and replace with a simple walkdir-based keyword
search. QMD never worked reliably in production, added bundle bloat,
and showed "Index failed" in the status bar.

What changed:
- Rust: deleted indexing.rs, rewrote search.rs as pure keyword search
- Frontend: removed useIndexing hook, IndexingBadge, hybrid search
- Menu: removed "Reindex Vault" command from palette and menu bar
- Config: removed qmd from bundle resources and build script
- Deleted tools/qmd/ (QMD CLI tool, MCP server, tests)
- Updated docs (ARCHITECTURE, ABSTRACTIONS, GETTING-STARTED)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useMenuEvents.test.ts

@@ -33,7 +33,6 @@ function makeHandlers(): MenuEventHandlers {
     onResolveConflicts: vi.fn(),
     onViewChanges: vi.fn(),
     onInstallMcp: vi.fn(),
-    onReindexVault: vi.fn(),
     onReloadVault: vi.fn(),
     onReopenClosedTab: vi.fn(),
     onOpenInNewWindow: vi.fn(),
@@ -296,12 +295,6 @@ describe('dispatchMenuEvent', () => {
     expect(h.onInstallMcp).toHaveBeenCalled()
   })
 
-  it('vault-reindex triggers reindex vault', () => {
-    const h = makeHandlers()
-    dispatchMenuEvent('vault-reindex', h)
-    expect(h.onReindexVault).toHaveBeenCalled()
-  })
-
   it('vault-reload triggers reload vault', () => {
     const h = makeHandlers()
     dispatchMenuEvent('vault-reload', h)