Add a checklist of pending actions to the PR bot comment

When a skill PR is submitted, the PR bot should post (or update) a comment that includes a checklist of all pending actions and validation steps. This gives PR authors and reviewers a clear, at-a-glance view of what's been completed and what's still outstanding.

It would be great if there was a command that the PR author can use to update the checklist as needed. If we are able to create such a command, we should show the command to update the checklist.

cc @ManishJayaswal