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/services/sqlite/SessionStore.ts

@@ -616,6 +616,33 @@ export class SessionStore {
     return stmt.all(sdkSessionId) as any[];
   }
 
+  /**
+   * Get observations by array of IDs with ordering and limit
+   */
+  getObservationsByIds(
+    ids: number[],
+    options: { orderBy?: 'date_desc' | 'date_asc'; limit?: number } = {}
+  ): any[] {
+    if (ids.length === 0) return [];
+
+    const { orderBy = 'date_desc', limit } = options;
+    const orderClause = orderBy === 'date_asc' ? 'ASC' : 'DESC';
+    const limitClause = limit ? `LIMIT ${limit}` : '';
+
+    // Build placeholders for IN clause
+    const placeholders = ids.map(() => '?').join(',');
+
+    const stmt = this.db.prepare(`
+      SELECT *
+      FROM observations
+      WHERE id IN (${placeholders})
+      ORDER BY created_at_epoch ${orderClause}
+      ${limitClause}
+    `);
+
+    return stmt.all(...ids) as any[];
+  }
+
   /**
    * Get summary for a specific session
    */