## Why

Tool instances (code-server, jupyter-notebook) run inside Docker containers with internal network addresses. Currently the "Open" button links to `http://localhost:{port}`, which only works from inside the API container and fails when opened from the user's browser. We need a way to expose these instances to users over HTTPS.

## What Changes

- Add a proxy endpoint to the backend API: `/instances/{id}/proxy/{path:path}`
- Proxy requests from the API to the running container (via docker network or internal IP)
- Update frontend "Open" button to use the proxy URL instead of `localhost`
- Add WebSocket proxy support for real-time features (terminal already uses WebSocket)
- Ensure only the instance owner can access the proxied content

## Capabilities

### New Capabilities

- `instance-proxy`: HTTP proxying for running tool instances through the API

### Modified Capabilities

- None (this is purely an infrastructure/transport feature, not a change to existing capability requirements)

## Impact

- Backend: New proxy endpoint, container network discovery, request forwarding
- Frontend: Update instance "Open" link to use proxy URL
- Docker: Containers must be reachable from API container (already true via docker network)
- Security: Owner-only access enforced at proxy level