Plan mode: Add "Save plan only" option to exit dialog

Feature Request

When exiting plan mode, the current dialog presents three options:

• Auto accept edits
• Yes, manually approve edits
• Tell Claude what to change

There is no way to simply save the plan and stop without triggering implementation. The plan file is already written to disk at this point, but users are forced to either pick an implementation path or use Esc (which isn't obvious).

Proposed Change

Add a 4th option to the plan exit dialog:

Save plan only — Keep the plan as-is, don't implement anything yet.

This would dismiss the dialog without Claude proceeding to implement, leaving the plan file saved for later reference.

Use Case

Users often want to:

• Review the plan with teammates before implementing
• Save the plan as documentation without immediate implementation
• Stage the plan for a future session

Currently the only workaround is pressing Esc (undocumented) or selecting option 3 and typing "stop here".