feat: replace Docker widget sandboxing with Secure Exec (V8 isolates)

[homanp]

What

Replace the Docker-based widget build and serve system with Secure Exec — a V8 isolate-based secure Node.js execution library. Each widget now runs in its own NodeRuntime sandbox instead of a shared Docker container.

Architecture:

1. Shared base template — On first widget build, a shared base directory is created with all npm deps + shadcn/ui components pre-installed. This is reused across all widget sandboxes.
2. Per-widget sandbox — Each widget gets a temp directory copied from the base template. The AI agent writes code into it, Vite builds the widget, and a SecureExec NodeRuntime serves the built files via HTTP.
3. Proxy route — The Next.js API route proxies iframe requests to the sandbox's HTTP server.

Changes:

• src/lib/widget-runner.ts — Complete rewrite. Each widget sandbox:
  • Copies the shared base template (with all deps + shadcn pre-installed)
  • Agent writes widget code into the sandbox directory
  • vite build runs in the sandbox to produce static assets
  • A SecureExec NodeRuntime serves the built files via an HTTP file server
• src/app/api/widget/[id]/[[...path]]/route.ts — Updated proxy route to use fetchFromWidget() which fetches from the sandbox's HTTP server
• src/app/api/chat/route.ts — Updated system prompt to reference Secure Exec instead of Docker
• next.config.ts — Added secure-exec and @secure-exec/* to serverExternalPackages, removed dockerode
• package.json — Added secure-exec, removed dockerode and @types/dockerode
• .github/workflows/ci.yml — Updated Node version from 20 to 22 (required for isolated-vm native prebuilds)
• AGENTS.md — Updated to reflect new architecture (no Docker required)

Why

Secure Exec provides lightweight, fast V8 isolate-based sandboxing that eliminates the Docker dependency:

• No infrastructure required — No Docker daemon, no container images, no volume management
• ~176x faster cold start than container-based sandboxes
• ~75x less memory per widget instance (~3.4 MB vs ~256 MB container minimum)
• Full Node.js compatibility — fs, child_process, http, npm all work inside the isolate
• Deny-by-default permissions — Granular control over filesystem, network, and process access

Test plan

• Tests pass locally (npm test — 42/42 passing)
• Lint passes (npm run lint — 0 errors)
• TypeScript compiles cleanly (npx tsc --noEmit)
• Production build succeeds (npm run build)
• Tested manually — created a counter widget end-to-end, verified interactive functionality