fix: only apply interim fallback when no tools were used

Addresses Codex review: if the model sent interim text then used tools,
the interim text should not be used as fallback for the final response.

nanobot/agent/loop.py

@@ -245,7 +245,8 @@ class AgentLoop:
                     final_content = None
                     continue
                 # Fall back to interim content if retry produced nothing
-                if not final_content and interim_content:
+                # and no tools were used (if tools ran, interim was truly interim)
+                if not final_content and interim_content and not tools_used:
                     final_content = interim_content
                 break