chore(ci): make country-map drift comment sticky

Every push to a country-map PR retriggers the regen workflow, which
was stacking a fresh drift warning comment on every commit (15+ on
this branch alone). Switch to the sticky-comment pattern:

- Tag each drift comment with a hidden HTML marker
- On drift: look up the existing tagged comment by ID and edit it in
  place, only posting a new one if none exists
- On drift cleared: delete the existing tagged comment so the PR
  doesn't carry a stale warning

Cleaned up the existing 15 stacked comments out of band.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>