feat(hh): P3 — summonable sandbox + shared PTY ⛧

Collaborative sandbox over the same zero-knowledge encrypted channel:
- sbx.rs: SandboxBackend (Local / Docker / Multipass) spawning a shell in a PTY
  (portable-pty); reader thread pumps output to the broker.
- Broker (owner's client): /sbx launch [backend] [image] boots the sandbox and
  relays PTY output as encrypted {"_sbx":"data"} frames; /sbx stop tears down.
  PTY input arrives as {"_sbx":"input"} frames and is written back.
- All clients render the shared terminal from data frames via a vt100 parser;
  F2 toggles drive mode (keystrokes -> input frames, incl. Ctrl-C); esc releases.
- ui.rs: sandbox pane (split below chat) with drive indicator.
- Server stays zero-knowledge: PTY bytes are Fernet-encrypted like chat/files;
  the VM runs on the initiator's client, never the server.

Tests (cargo test, 4 pass): PTY I/O round-trip + headless end-to-end relay
(PTY -> _sbx frame encode -> decode -> vt100 screen shows command output).

Note: Multipass assumes the instance is launched separately (lifecycle = P3b);
per-user unix accounts + sudo delegation = P4.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hh/src/main.rs

@@ -7,6 +7,7 @@
 mod app;
 mod crypto;
 mod net;
+mod sbx;
 mod theme;
 mod ui;