Move the inline programs out of the update-changelog skill into
scripts/changelog.py (check-merged, cross-ref, report) and shrink
the skill to the workflow alone.
Teach scripts/gh.py to retry transient HTTP 504s, resolve lookups
in batches of 50 (new issue subcommand, milestone on prs output),
and report 💥 entries without the breaking change label.
Add the nine missing 2.17.0 changelog entries found while
trying the new flow.
AI-assisted-by: muse-spark-1.3
