Write or update a handoff document so the next agent with fresh context can continue this work.

Steps:

1. Check if HANDOFF.md already exists in the project
2. If it exists, read it first to understand prior context before updating
3. Create or update the document with:
   • Goal: What we're trying to accomplish
   • Current Progress: What's been done so far
   • What Worked: Approaches that succeeded
   • What Didn't Work: Approaches that failed (so they're not repeated)
   • Next Steps: Clear action items for continuing

Save as HANDOFF.md in the project root and tell the user the file path so they can start a fresh conversation with just that path.