Revise unattended work instructions in copilot-instructions.md

[nohwnd]

Revised unattended work instructions based on review feedback on #15531:

• Don't require Release builds locally — only as a troubleshooting step
• Use --force-with-lease instead of plain force-push
• Check git remote -v to identify fork vs upstream
• Check PR status within 15-20 min (Windows finishes first)
• Reason about automated review comments (they can be wrong)
• Don't create draft PRs (undrafting forces re-build)
• Mark green PRs as ready to merge

Clarifies #15531