fix: write final assistant message to permanent chat log

The streaming widget shows content during streaming then hides on
completion, but nobody was writing the final response to the RichLog.
The assistant message flashed briefly then disappeared.

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

app/agent/loop.py

@@ -161,6 +161,8 @@ class AgentLoop:
 
             # No tool calls → task complete (plain text response)
             if not assistant_msg.tool_calls:
+                if self._display and assistant_msg.content:
+                    self._display.write_assistant_message(assistant_msg.content)
                 break
 
             # Execute tool calls