Plan file hygiene / cleanup skill sessions accumulate stray PLAN.md, scratch files with no consolidation

Over long running agentic sessions (e.g. Claude Code, or others AI ), each planning or research step tends to spawn its own
PLAN.md, NOTES.md, or scratch markdown file. Over time these accumulate in
the repo/workdir with no lifecycle: nothing merges them, prunes stale ones,
or distinguishes "still relevant" from "superseded by a later plan."

Proposing a skill that would:
- Detect existing planning/scratch .md files in the working directory
before creating a new one
- Offer to merge/update an existing plan file instead of spawning a new one
- Flag or archive stale plan files once their associated task is completed

This would reduce repo clutter and make it easier to find the "current"
plan when returning to a project after a break.

Happy to take a first pass at implementing this myself if the idea sounds
useful would appreciate feedback on scope/approach before I start.