Feature request: Auto-clear active plan on completion

Feature Request

Problem: When a plan is completed, it remains "active" and gets picked up by new sessions. The .md plan files are useful to keep around for reference, but having stale completed plans loaded as active in new sessions is undesirable.

Desired behavior: Automatically deactivate/archive a plan when it's completed, so it no longer appears as the active plan in subsequent sessions — while keeping the .md file intact for reference.

Possible solutions:

1. A PlanCompleted hook event that fires when all plan steps are done, allowing users to archive/move the plan file
2. A setting like autoArchiveCompletedPlans: true that moves completed plans out of the active plans directory
3. Marking plan files with a "completed" status in their frontmatter so they're skipped when loading active plans

Current workarounds: None are satisfactory. The Stop hook fires on every session end (not just plan completion), and manual cleanup defeats the purpose.