feat: persistent chat history with clear button

- Add dashboard/store.py: MessageLog dataclass singleton tracking
  user/agent/error messages for the lifetime of the server process
- agents.py: write each chat turn to MessageLog; add GET and DELETE
  /agents/timmy/history routes returning the history.html partial
- partials/history.html: render stored messages by role (YOU / TIMMY /
  SYSTEM); falls back to the Mission Control init message when empty
- index.html: chat-log loads history via hx-get on page start; new
  CLEAR button in panel header sends hx-delete to reset the log
- style.css: add .mc-btn-clear (muted, red-on-hover for the header)
- tests: autouse reset_message_log fixture in conftest; 5 new history
  tests covering empty state, recording, offline errors, clear, and
  post-clear state → 32 tests total, all passing

https://claude.ai/code/session_01KZMfwBpLuiv6x9GbzTqbys

tests/conftest.py

@@ -18,6 +18,15 @@ for _mod in [
     sys.modules.setdefault(_mod, MagicMock())
 
 
+@pytest.fixture(autouse=True)
+def reset_message_log():
+    """Clear the in-memory chat log before and after every test."""
+    from dashboard.store import message_log
+    message_log.clear()
+    yield
+    message_log.clear()
+
+
 @pytest.fixture
 def client():
     from dashboard.app import app