Feature request: expose plan_mode and sandbox in statusline JSON

Feature Request

Add plan_mode and sandbox fields to the JSON object piped to statusline commands via stdin.

Current statusline JSON schema

The statusline command currently receives these fields:

{
  "session_id": "...",
  "model": { "id": "claude-opus-4-6", ... },
  "context_window": {
    "total_input_tokens": ...,
    "total_output_tokens": ...,
    ...
  },
  "cost": {
    "total_cost_usd": ...
  }
}

Requested additions

plan_mode (boolean)

Whether the session is currently in plan mode (/plan toggled on). This would allow statusline scripts to display the current planning state.

sandbox (boolean or object)

Whether sandbox mode is active. Could be a simple boolean, or an object with details like { "enabled": true, "network": {...}, "filesystem": {...} }.

Use case

I use a custom statusline script to display session state at a glance. Currently there is no way to detect plan mode or sandbox status from the statusline command — the JSON stdin doesn't include these fields, and there are no corresponding environment variables (CLAUDE_PLAN_MODE, CLAUDE_SANDBOX, etc.).

Without these fields, any plan/sandbox indicator in the statusline is always stale (e.g., always shows "off").

Example desired output

Month: $4.23  sandbox:on  plan:on

Thank you\!