feat(ci): allow scheduled posts with DO NOT MERGE instruction (#1129)
Add support for PRs with future-dated content when the PR body contains
"DO NOT MERGE until YYYY-MM-DD UTC". The validation will pass and post
a warning comment listing the scheduled files and their dates.
- Add @octokit/rest for GitHub API interaction
- Parse PR body for merge-until instruction
- Allow future dates up to and including the merge-until date
- Post warning comment with list of scheduled files
- Fail if post date exceeds the merge-until date