feat(sbx,ui): VM snapshot save/load + collapsible clustered help menu

- /sbx save|load|snaps: docker commit → hh-snap:<label> image that
  survives /sbx stop; load relaunches a fresh sandbox from it; multipass
  delegates to `multipass snapshot`. Local backend unsupported.
- Help overlay redesigned into topical clusters (SANDBOX, AI AGENTS,
  PERMISSIONS, FILES, APPEARANCE, KEYS, ROSTER GLYPHS), collapsed by
  default; up/down highlight a cluster, left/right/Enter expand-collapse
  it (tmux-style), PgUp/PgDn scroll overflow, Esc closes.
- docstring: example uses --model qwen2.5:3b (the locally-pulled model),
  not llama3.

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

cmd_chat/agent/__main__.py

@@ -4,7 +4,7 @@ Examples
 --------
     # local Ollama (default, recommended)
     python -m cmd_chat.agent 127.0.0.1 3000 --name oracle \
-        --password hunter2 --model llama3 --no-tls
+        --password hunter2 --model qwen2.5:3b --no-tls
 
     # cloud, opt-in
     python -m cmd_chat.agent 127.0.0.1 3000 --name claude \