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/state/sessions.tsx

@@ -0,0 +1,41 @@
+import { createContext, useContext, useState, type ReactNode } from "react";
+
+export interface Session {
+  id: string;
+  display_name: string;
+  tool_type_name: string;
+  tool_icon: string;
+  repository_name: string;
+  project_name: string;
+  status: string;
+  url: string | null;
+}
+
+interface SessionsContextType {
+  sessions: Session[];
+  setAllSessions: (sessions: Session[]) => void;
+}
+
+const SessionsContext = createContext<SessionsContextType | undefined>(undefined);
+
+export const SessionsProvider = ({ children }: { children: ReactNode }) => {
+  const [sessions, setSessions] = useState<Session[]>([]);
+
+  const setAllSessions = (newSessions: Session[]) => {
+    setSessions(newSessions);
+  };
+
+  return (
+    <SessionsContext.Provider value={{ sessions, setAllSessions }}>
+      {children}
+    </SessionsContext.Provider>
+  );
+};
+
+export const useSessions = () => {
+  const context = useContext(SessionsContext);
+  if (context === undefined) {
+    throw new Error("useSessions must be used within a SessionsProvider");
+  }
+  return context;
+};