Fix two startup errors for Linux/Docker deployment

1. agents SDK shadowing: switch from hardcoded Windows path
(.venv/Lib/site-packages) to sysconfig.get_path("purelib")
so the fix works on Linux inside the container.

2. cache package missing: .dockerignore excluded cache/ entirely
but cache/__init__.py and cache/query_cache.py are source code.
Now only cache/query_store/ (runtime data) is excluded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.dockerignore

@@ -16,4 +16,4 @@ server.err
 COST_NOTES.md
 documents/
 vector_store/
-cache/
+cache/query_store/