[FEATURE] Include sandbox runtime state in statusline JSON input

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Feature Request: Include sandbox runtime state in statusline JSON input

The statusline command receives session data (model, cost, context, duration, etc.) as JSON input. However, the current sandbox enabled/disabled state is
missing from this data.

The /sandbox command toggles sandbox per-session without modifying settings.json, so statusline scripts that read sandbox.enabled from the settings file
show stale values. Other dynamic fields (cost, context, duration) update in real-time, but sandbox status does not.

Proposed Solution

Add a sandbox.enabled boolean to the statusline input JSON reflecting the actual runtime state.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

I want to have realtime feedback wether the sandbox is enabled or not

Additional Context

_No response_