fix: add null guards for getChromaSync() when Chroma is disabled (#1336)

When CLAUDE_MEM_CHROMA_ENABLED=false, getChromaSync() returns null.
Two call sites were missing null guards, causing "null is not an object"
errors on every UserPromptSubmit / session init.

Fixes #1294

Vibe-coded by Ousama Ben Younes

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/services/worker/SearchManager.ts

@@ -61,6 +61,9 @@ export class SearchManager {
     limit: number,
     whereFilter?: Record<string, any>
   ): Promise<{ ids: number[]; distances: number[]; metadatas: any[] }> {
+    if (!this.chromaSync) {
+      return { ids: [], distances: [], metadatas: [] };
+    }
     return await this.chromaSync.queryChroma(query, limit, whereFilter);
   }