Add sandbox mode/status to statusline data fields

Problem

The statusline JSON data includes model info, git state, token counts, cost, rate limits, and more — but there's no field indicating the current sandbox mode (Docker sandbox, local sandbox, or no sandbox).

As a user, I'd like to see at a glance whether my session is sandboxed or not, especially when switching between projects with different permission configurations.

Suggested improvement

Add a field like sandbox.mode (or execution_environment) to the statusline JSON payload, with values such as docker, local, or none. This would allow statusline scripts to display the current sandbox status alongside other session info.

Context

All other important session state is already exposed to the statusline — sandbox mode is a notable gap, especially since it directly affects what commands can do to the host system.