Implement hybrid search: Chroma semantic + SQLite temporal

Core implementation:
- Added Chroma MCP client integration to search-server.ts
- Implemented queryChroma() helper with Python dict parsing
- Added VECTOR_DB_DIR constant to paths.ts
- Added SessionStore.getObservationsByIds() method

Search handlers updated:
- search_observations: Semantic-first with 90-day temporal filter
- find_by_concept/type/file: Metadata-first, semantic-enhanced ranking
- All handlers fall back to FTS5 if Chroma unavailable

Technical details:
- Direct MCP client usage (no abstractions)
- Regex parsing of Chroma Python dict responses
- Semantic ranking preserved in final results
- Graceful degradation to FTS5-only search

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/shared/paths.ts

@@ -32,6 +32,7 @@ export const TRASH_DIR = join(DATA_DIR, 'trash');
 export const BACKUPS_DIR = join(DATA_DIR, 'backups');
 export const USER_SETTINGS_PATH = join(DATA_DIR, 'settings.json');
 export const DB_PATH = join(DATA_DIR, 'claude-mem.db');
+export const VECTOR_DB_DIR = join(DATA_DIR, 'vector-db');
 
 // Claude integration paths
 export const CLAUDE_SETTINGS_PATH = join(CLAUDE_CONFIG_DIR, 'settings.json');