1.2.0

1.2.0: run in background, cloud sandboxes, and a clearer model picker

- Run in background: after 10 seconds a command the agent waits for can move
  to the background jobs and keep running, on this computer or in a cloud
  session's sandbox.
- Cloud sessions: background commands in the sandbox, managed like local
  ones; a command can stop itself after a time limit (stop_after).
- A Cloud sandboxes tab: specs (how a sandbox is built), the sandboxes running
  now, and snapshots. A cloud session built from a spec gets a folder of its
  own in the sidebar.
- Model picker: intelligence, speed and cost bars, and Unlimited instead of
  credits for a model the plan includes.
- Error cards: a refusal's link is a button; a provider's raw error reads as
  what happened and what to do.
- Fix: fs_glob with ".." in its pattern listed files outside the project
  folder without asking.
- Fix: a crash while saving state.json could lose every session's folder.
- Fix: command output no longer garbles a character split across chunks.
- Fix: a sub-agent's waiting approval stays on screen and clickable.
- Fix: a restarted local MCP server is no longer cut off by its old process;
  browser sign-in keeps waiting through a 429; undo says when it cannot put
  back a binary file.