Add a /signoff command #260

The DCO is a good idea, but sometimes creates friction. For some PRs, I want to make it easy for a maintainer to just type /signoff <git short sha> and this would inside a GHA:

• Verify the commenter has triage role or greater
• Check out the PR and verify the tip commit has that sha (if not bail and add a comment to the PR)
• Add signoff based on the github identity of the commenter
• force push

Agent: Ensure you do a search for existing actions that may fit here, but prioritize things used by well-known projects if pulling in an existing action workflow; consider provenance/maintenance of any used actions