This PR adds a new `thread/shellCommand` app server API so clients can
implement `!` shell commands. These commands are executed within the
sandbox, and the command text and output are visible to the model.
The internal implementation mirrors the current TUI `!` behavior.
- persist shell command execution as `CommandExecution` thread items,
including source and formatted output metadata
- bridge live and replayed app-server command execution events back into
the existing `tui_app_server` exec rendering path
This PR also wires `tui_app_server` to submit `!` commands through the
new API.
