[Requests and ideas] Allow Plan to be collapsible

### Feature summary

Allow Plan to be collapsible so that I have more screen real estate to read the agent's response to my questions.

### What problem are you trying to solve?

In plan mode, can we add an UI element to collapse the plan?

Use case:
- I'm defining scope for a feature using plan mode
- When provided with options, I opt for the "Suggest changes to the plan...". When doing so, I ask a question ending with "[more context]...help me rationalize whether to include this in Phase 1 or 2"
- Copilot generates a markdown response in the chat window, but also opens plan mode with its recommendation

Papercut:
- I don't want to look at the plan yet; I want to read the response. However, the plan takes 50% of the screen real estate.
- It's a poor experience to scroll with 50% of vertical space dedicated to the agent's repsonse to my scoping question.

### Proposed solution

Simple UI element.

### Workflow impact

Just annoying. I'm not going to continue with the Plan until I read the agent's response to my question to help me rationalize scoping questions.

### Installation context

_No response_

### Additional context

_No response_