Expose current plan name and file path in status line JSON data

Feature Request

Summary: Add plan name and plan file path to the JSON data sent to status line scripts, so users can display the current plan in their status line and make it clickable to jump to the file.

Motivation

When working in plan mode, it would be very helpful to always see the current plan name in the status line. This provides at-a-glance context about what the session is working on. Making it clickable (via OSC 8 hyperlinks) would let users jump directly to the plan file.

Current Behavior

The status line JSON includes:

• Model info
• Working directory
• Context window usage
• Cost/duration tracking
• Rate limits
• Worktree info
• Vim mode
• Agent name
• Session ID / transcript path

Plan information is not included.

Proposed Behavior

Add the following fields to the status line JSON when a plan is active:

{
  "plan": {
    "name": "auth-middleware-refactor",
    "file_path": "/path/to/plan-file.md"
  }
}

This would allow status line scripts to render something like:

📋 auth-middleware-refactor  |  opus-4  |  $0.12

With the plan name wrapped in an OSC 8 hyperlink pointing to the plan file for terminal click-to-open support.

Use Case

Users who rely on the status line for session awareness want to see which plan is guiding the current session without having to check manually (e.g., Ctrl+G).