Plan mode: add option to approve plan without executing

Problem

When using plan mode, the only option after reviewing a plan is "approve and begin executing." There's no way to say "yes, the plan looks good" without immediately starting implementation.

This creates a frustrating loop when the goal is to finalize a plan document for review (e.g., writing a plan file to share with a colleague) rather than to start coding.

Expected Behavior

Add an option like "Approve plan and exit without executing" — accept the plan, exit plan mode, and stop. No implementation begins.

Use Case

• Writing a plan file (e.g., PLAN.md) to review with a teammate before any code is written
• Using plan mode purely for the planning/exploration phase, with implementation deferred to a later session
• Any workflow where the plan itself is the deliverable

Current Workaround

None cleanly. Approving always triggers execution, and rejecting discards the plan. The user has to approve and then interrupt execution, which is clunky.