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/components/app-shell.tsx

@@ -1,7 +1,11 @@
+import { useCallback, useEffect } from "react";
 import { Link, NavLink, Outlet } from "react-router-dom";
 
+import { getUserSessions } from "../api/sessions";
+import type { Session } from "../api/sessions";
 import { useTheme } from "../hooks/use-theme";
 import { useAuth } from "../state/auth";
+import { useSessions } from "../state/sessions";
 import { Icon } from "./icon";
 import type { IconName } from "../utils/icons";
 
@@ -13,9 +17,46 @@ const NAV_ITEMS: { to: string; label: string; icon: IconName }[] = [
   { to: "/settings", label: "Settings", icon: "settings" }
 ];
 
+const SessionItem = ({ session }: { session: Session }) => {
+  const isRunning = session.status === "running";
+  
+  return (
+    <a
+      href={session.url || "#"}
+      target="_blank"
+      rel="noopener noreferrer"
+      className="nav-item session-item"
+      title={`${session.display_name} (${session.status})`}
+    >
+      <span className={`session-status ${isRunning ? "running" : ""}`} />
+      <Icon name={session.tool_icon as IconName} size="sm" />
+      <span className="session-name">{session.display_name}</span>
+    </a>
+  );
+};
+
 export const AppShell = () => {
   useTheme();
   const { user, logout } = useAuth();
+  const { sessions, setAllSessions } = useSessions();
+
+  const loadSessions = useCallback(async () => {
+    try {
+      const data = await getUserSessions();
+      setAllSessions(data);
+    } catch {
+      // Silently fail - sessions are optional
+    }
+  }, [setAllSessions]);
+
+  useEffect(() => {
+    void loadSessions();
+    // Poll every 10 seconds
+    const interval = setInterval(() => {
+      void loadSessions();
+    }, 10000);
+    return () => clearInterval(interval);
+  }, [loadSessions]);
 
   return (
     <div className="shell">
@@ -53,6 +94,16 @@ export const AppShell = () => {
               {item.label}
             </NavLink>
           ))}
+          
+          {sessions.length > 0 && (
+            <>
+              <div className="nav-divider" />
+              <div className="nav-section-title">Sessions</div>
+              {sessions.map((session) => (
+                <SessionItem key={session.id} session={session} />
+              ))}
+            </>
+          )}
         </aside>
 
         <main className="shell-content">