Add WebSocket-based streaming exec endpoint and streaming file
upload/download endpoints to the control plane API. Includes new
host agent RPC methods (ExecStream, StreamWriteFile, StreamReadFile),
envd client streaming support, and OpenAPI spec updates.
- REST API (chi router): sandbox CRUD, exec, pause/resume, file write/read
- PostgreSQL persistence via pgx/v5 + sqlc (sandboxes table with goose migration)
- Connect RPC client to host agent for all VM operations
- Reconciler syncs host agent state with DB every 30s (detects TTL-reaped sandboxes)
- OpenAPI 3.1 spec served at /openapi.yaml, Swagger UI at /docs
- Added WriteFile/ReadFile RPCs to hostagent proto and implementations
- File upload via multipart form, download via JSON body POST
- sandbox_id propagated from control plane to host agent on create
