Fix agents SDK shadowing on Linux (cross-platform sys.path fix)

Windows used .venv/Lib/site-packages (capital L); Linux needs
.venv/lib/python3.11/site-packages. Switch to sysconfig.get_path("purelib")
so the venv site-packages is correctly prioritised on both platforms.

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

agents/rag_agent.py

@@ -1,13 +1,15 @@
 import os
 import sys
+import sysconfig
 from pathlib import Path
 
-# The local backend/agents/ package shadows the openai-agents SDK `agents` package.
-# Fix: force .venv/Lib/site-packages to index 0, then clear the cached local entry.
-_venv_site = str((Path(__file__).parent.parent / ".venv" / "Lib" / "site-packages").resolve())
-if _venv_site in sys.path:
+# The local agents/ package shadows the openai-agents SDK which also installs as `agents`.
+# Fix: move the venv site-packages to front of sys.path (works on Windows + Linux).
+_venv_site = sysconfig.get_path("purelib")
+if _venv_site and _venv_site in sys.path:
     sys.path.remove(_venv_site)
-sys.path.insert(0, _venv_site)
+if _venv_site:
+    sys.path.insert(0, _venv_site)
 for _key in [k for k in sys.modules if k == "agents" or (k.startswith("agents.") and k != __name__)]:
     del sys.modules[_key]