fix: use correct hook field name last_assistant_message

The Claude Code Stop hook sends `last_assistant_message`, not
`assistant_message`. This was causing response_chars=0 on all
captured interactions. Also removes the temporary debug log block.

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

deploy/hooks/capture_stop.py

@@ -59,8 +59,9 @@ def _capture() -> None:
         return
 
     hook_data = json.loads(raw)
+
     session_id = hook_data.get("session_id", "")
-    assistant_message = hook_data.get("assistant_message", "")
+    assistant_message = hook_data.get("last_assistant_message", "")
     transcript_path = hook_data.get("transcript_path", "")
     cwd = hook_data.get("cwd", "")