feat: add Google code retrieval across desktop CLI and MCP

AGENTS.md

@@ -16,6 +16,10 @@ readiness without completed release gates and recorded evidence.
 - Use bounded work, explicit errors, versioned storage, and offline defaults.
 - Never substitute simulated embeddings for working semantic search.
 - Production semantic search must use Google EmbeddingGemma 2 as in DigUp.
+  Code scope uses Google's CodeRetrieval query prefix; query-vector cache keys
+  must distinguish code and general search tasks. Preserve exact source bytes,
+  line locations and code filtering before all candidate limits. Re-run actual
+  CPU code fixtures after changes to tokenizer offsets or code extraction.
   MiniLM proof artifacts are historical experiments, not an accepted backend.
 - The desktop UI is the primary product, including local model setup and semantic
   indexing/search. Keep a separate CLI-only build for headless Linux with no GUI dependencies.