[FEATURE] Add `/plan done` command to detach active plan from context without `/clear`

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

After completing a plan implementation, the active plan file continues to be injected into context at every turn, consuming tokens unnecessarily for any subsequent unrelated work.

Also, writing a new plan means wiping the existing file to start over, which is not ideal either and looses the plans history.

Proposed Solution

A command like /plan done or /plan detach, or a menu option when the plan is complete that:

• Stops injecting the plan file into context at each turn
• Removes the plan name pill from the status bar
• Does not clear conversation history
• When starting a new /plan, creates a new file (with timestamp or index) instead of overwriting the previous one, preserving plan history

Alternative Solutions

The only current workaround is /clear, which wipes the entire conversation context, losing all session history.

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

1. Enter plan mode (/plan), write a plan, validate it (with "auto-accept edits" for instance)
2. Implement the plan, tests pass, commit done
3. Continue working on subsequent tasks in the same session
4. The previous plan is still actively injected into context at every turn, and the blue plan pill is still visible in the status bar

Additional Context

• Plans can be thousands of tokens injected at every turn. On long sessions with multiple sequential tasks, this is significant wasted context.
• Users working on multiple tasks in one session currently have no choice between "keep burning tokens on a finished plan" and "lose all context".
• Plan history is destroyed when starting a new plan in the same session (the previous file is overwritten).

Related issues

• #37484 — Plan mode name pill persists in status bar after ExitPlanMode
• #34782 — Plan from previous session auto-injected and executed in new session
• #32117 — [FEATURE] /implement command to clear context and start implementation from a plan file