feat: implement tool instances backend and session navigation

Backend:
- Create ToolInstance model with status tracking
- Add Alembic migration for tool_instances table
- Create Docker service for compose template rendering and container execution
- Add CRUD API endpoints for tool instances
- Add lifecycle endpoints (start/stop/restart)
- Add user sessions endpoint for navigation
- Register routers in main.py

Frontend:
- Create SessionsProvider with React context
- Create sessions API client
- Update AppShell with sessions section in navigation
- Add session status indicators and polling
- Add CSS for session navigation

Quality gates: typecheck ✓, lint ✓, build ✓

apps/web/src/main.tsx

@@ -4,13 +4,16 @@ import { BrowserRouter } from "react-router-dom";
 
 import { AppRouter } from "./router";
 import { AuthProvider } from "./state/auth";
+import { SessionsProvider } from "./state/sessions";
 import "./styles.css";
 
 ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
     <BrowserRouter>
       <AuthProvider>
-        <AppRouter />
+        <SessionsProvider>
+          <AppRouter />
+        </SessionsProvider>
       </AuthProvider>
     </BrowserRouter>
   </React.StrictMode>