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

src/dashboard/routes/agents.py

@@ -6,6 +6,7 @@ from fastapi.responses import HTMLResponse
 from fastapi.templating import Jinja2Templates
 
 from timmy.agent import create_timmy
+from dashboard.store import message_log
 
 router = APIRouter(prefix="/agents", tags=["agents"])
 templates = Jinja2Templates(directory=str(Path(__file__).parent.parent / "templates"))
@@ -27,6 +28,25 @@ async def list_agents():
     return {"agents": list(AGENT_REGISTRY.values())}
 
 
+@router.get("/timmy/history", response_class=HTMLResponse)
+async def get_history(request: Request):
+    return templates.TemplateResponse(
+        request,
+        "partials/history.html",
+        {"messages": message_log.all()},
+    )
+
+
+@router.delete("/timmy/history", response_class=HTMLResponse)
+async def clear_history(request: Request):
+    message_log.clear()
+    return templates.TemplateResponse(
+        request,
+        "partials/history.html",
+        {"messages": []},
+    )
+
+
 @router.post("/timmy/chat", response_class=HTMLResponse)
 async def chat_timmy(request: Request, message: str = Form(...)):
     timestamp = datetime.now().strftime("%H:%M:%S")
@@ -40,6 +60,12 @@ async def chat_timmy(request: Request, message: str = Form(...)):
     except Exception as exc:
         error_text = f"Timmy is offline: {exc}"
 
+    message_log.append(role="user", content=message, timestamp=timestamp)
+    if response_text is not None:
+        message_log.append(role="agent", content=response_text, timestamp=timestamp)
+    else:
+        message_log.append(role="error", content=error_text, timestamp=timestamp)
+
     return templates.TemplateResponse(
         request,
         "partials/chat_message.html",