feat: add web terminal UI with WebSocket console and clone management API

Introduces a browser-based terminal interface backed by xterm.js, served
directly from the binary via an embedded HTML asset.

New HTTP server (`serve [addr]`, default :8080):
  GET  /              — xterm.js terminal UI; ?id=N selects a clone
  GET  /clones        — JSON list of running clone IDs
  POST /clones        — spawn a new clone; returns {"id": N}
  DELETE /clones/{id} — destroy a clone by ID
  GET  /ws/{id}       — WebSocket console for clone {id}
                        binary frames = raw PTY I/O
                        text frames   = JSON resize {"rows":N,"cols":M}

Supporting changes:
- orchestrator: add SpawnSingle() and KillClone(id) for per-clone lifecycle
  management from the HTTP layer
- console: add a resize sideband Unix socket (console-resize.sock) that
  accepts newline-delimited JSON {"rows","cols"} messages and applies them
  to the PTY master via pty.Setsize; the WebSocket handler writes to this
  socket on text frames so browser window resizes propagate into the VM
- deps: add gorilla/websocket v1.5.3

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -27,6 +27,7 @@ require (
 	github.com/go-openapi/validate v0.22.0 // indirect
 	github.com/go-stack/stack v1.8.1 // indirect
 	github.com/google/uuid v1.3.0 // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
 	github.com/hashicorp/errwrap v1.0.0 // indirect
 	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/josharian/intern v1.0.0 // indirect