[FEATURE] open plan.md in viewer with shortcut `Ctrl+Y` in claude code cli terminal

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

I was using copilot cli recently, then I found it is quite convenient to view the plan file in a separate md viewer when the terminal cannot pin input box at the bottom now.

I can view the plan, scroll it to anywhere, without forgetting what I want to amend into the input box next.

Proposed Solution

Press Ctrl + Y to open the default markdown file viewer.

Alternative Solutions

Open markdown file viewer with /plan when the session is already in plan mode.

Priority

High - Significant impact on productivity

Feature Category

Interactive mode (TUI)

Use Case Example

1. Ask claude to make a complex plan
2. It gave you a long initial plan
3. You have to view it in the terminal
4. You have to remember what you want to change (in mind or in another file)
5. Scroll to the bottom to find the feedback input box
6. Type or paste your thought.

Additional Context

Use Ctrl + Y to open the plan.md when using plan mode in copilot cli terminal.