Agent Skills: pr-work-log

Surface off-PR work on the pull request as a maintained comment. Triggers when, while a PR is open, you do work tied to that PR that the diff does NOT show — dispatching/watching GitHub Actions runs, running a backfill/migration/data job, writing to a database, kicking off manual workflows, validation runs, or anything happening in CI/Actions/a DB/an external system. Keeps a single "Off-PR work log" comment up to date as each step lands.