When searching in FTS-only mode (no embedding provider), extract meaningful
keywords from conversational queries using LLM to improve search results.
Changes:
- New query-expansion module with keyword extraction
- Supports English and Chinese stop word filtering
- Null safety guards for FTS-only mode (provider can be null)
- Lint compliance fixes for string iteration
This helps users find relevant memory entries even with vague queries.
When no embedding provider is available (e.g., OAuth mode without API keys),
memory_search now falls back to FTS-only mode instead of returning disabled: true.
Changes:
- embeddings.ts: return null provider with reason instead of throwing
- manager.ts: handle null provider, use FTS-only search mode
- manager-search.ts: allow searching all models when provider is undefined
- memory-tool.ts: expose search mode in results
The search results now include a 'mode' field indicating 'hybrid' or 'fts-only'.
