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/utils/indexingHelpers.ts

@@ -1,10 +0,0 @@
-export function formatIndexedElapsed(lastIndexedTime: number | null): string {
-  if (!lastIndexedTime) return ''
-  const secs = Math.round((Date.now() - lastIndexedTime) / 1000)
-  if (secs < 60) return 'Indexed just now'
-  const mins = Math.floor(secs / 60)
-  if (mins < 60) return `Indexed ${mins}m ago`
-  const hrs = Math.floor(mins / 60)
-  if (hrs < 24) return `Indexed ${hrs}h ago`
-  return `Indexed ${Math.floor(hrs / 24)}d ago`
-}