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.sum

@@ -408,6 +408,8 @@ github.com/gorilla/mux v1.7.2/go.mod h1:1lud6UwP+6orDFRuTfBEV8e9/aOM/c4fVVCaMa2z
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 github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=
 github.com/gorilla/websocket v1.4.2/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
+github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
+github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/gregjones/httpcache v0.0.0-20180305231024-9cad4c3443a7/go.mod h1:FecbI9+v66THATjSRHfNgh1IVFe/9kFxbXtjV0ctIMA=
 github.com/grpc-ecosystem/go-grpc-middleware v1.0.0/go.mod h1:FiyG127CGDf3tlThmgyCl78X/SZQqEOJBCDaAfeWzPs=
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