docs(readme): rewrite database-is-locked troubleshooting for the bundled backend

The old entry described the retired multi-backend chain (better-sqlite3 → wasm
with npm-rebuild instructions). CodeGraph now bundles Node + node:sqlite (WAL),
so the lock class is gone. The entry now covers the only real cases: an old
pre-0.9 install (reinstall) or WAL disabled by the filesystem (move to local disk).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>