ci(country-map): regenerate GeoJSON outputs on config changes

New workflow `.github/workflows/country-map-build-regen.yml` that:

1. Triggers on PRs touching the build pipeline configs (or manually
   via workflow_dispatch).
2. Sets up Python 3.11 + Node 22 + PyYAML.
3. Runs `./scripts/build.sh` from a clean checkout.
4. Detects output drift (uncommitted changes in
   `superset/static/assets/country-maps/` or `src/data/manifest.json`).
5. If drift detected on a PR: leaves a comment telling the contributor
   to re-run the build locally and commit, then fails CI.

This forces "configs and outputs stay in sync" — a contributor can't
add a name override / region aggregation / composite without also
updating the committed GeoJSON outputs.

Tagged contents permissions only — does not auto-PR (which would
require deeper repo setup); contributors are expected to regenerate
locally and push.

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