Shepherd an already-open pull request to a mergeable state — poll CI, triage review comments, verify each one against the source, push fixes, and keep the branch current. Use when the user asks to "babysit", "monitor", "watch", "shepherd", or "drive" a PR, asks you to "fix the CI on my PR", "handle the review comments", "get this PR green", or "wait for checks and address feedback". Picks up where the `file-pr` skill leaves off. Does not approve, merge, or close anything.