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>

.github/workflows/country-map-build-regen.yml

@@ -63,14 +63,51 @@ jobs:
             echo "Outputs match committed snapshot — nothing to do."
           fi
 
-      - name: Comment on PR if drift
+      # Sticky comment: every push to a PR retriggers this workflow, so
+      # we'd otherwise stack a fresh drift comment on every commit.
+      # Look for an existing comment carrying our marker and edit it in
+      # place; only post a new one the first time.
+      - name: Comment on PR if drift (sticky)
         if: steps.drift.outputs.drift == 'true' && github.event_name == 'pull_request'
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+          COMMIT: ${{ github.event.pull_request.head.sha }}
         run: |
-          gh pr comment "$PR_NUMBER" --repo "${{ github.repository }}" --body \
-            "⚠️ Country Map: drift detected between locally committed GeoJSON outputs and CI-regenerated outputs. Either the YAML configs were updated without re-running \`./scripts/build.sh\`, OR mapshaper output differs cross-platform (macOS dev vs Linux CI). Investigate before merge."
+          MARKER="<!-- country-map-drift-bot -->"
+          BODY="$MARKER
+          ⚠️ **Country Map**: drift detected between the locally committed GeoJSON outputs and what CI just regenerated for commit \`${COMMIT:0:7}\`. Either the YAML configs were updated without re-running \`./scripts/build.sh\`, OR mapshaper output differs cross-platform (macOS dev vs Linux CI). Investigate before merge.
+
+          _This comment is sticky and updated on every push; old drift comments are not stacked._"
+
+          # Find an existing sticky comment by marker
+          EXISTING=$(gh api "repos/${REPO}/issues/${PR_NUMBER}/comments" \
+            --paginate --jq ".[] | select(.body | contains(\"${MARKER}\")) | .id" \
+            | head -n1)
+
+          if [ -n "$EXISTING" ]; then
+            gh api -X PATCH "repos/${REPO}/issues/comments/${EXISTING}" -f body="$BODY"
+          else
+            gh pr comment "$PR_NUMBER" --repo "$REPO" --body "$BODY"
+          fi
+
+      # If a previous push had drift but this one doesn't, remove the
+      # sticky comment so the PR doesn't carry a stale warning.
+      - name: Clear sticky comment if drift resolved
+        if: steps.drift.outputs.drift == 'false' && github.event_name == 'pull_request'
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          REPO: ${{ github.repository }}
+        run: |
+          MARKER="<!-- country-map-drift-bot -->"
+          EXISTING=$(gh api "repos/${REPO}/issues/${PR_NUMBER}/comments" \
+            --paginate --jq ".[] | select(.body | contains(\"${MARKER}\")) | .id" \
+            | head -n1)
+          if [ -n "$EXISTING" ]; then
+            gh api -X DELETE "repos/${REPO}/issues/comments/${EXISTING}"
+          fi
 
       # Informational only: cross-platform mapshaper output reproducibility
       # is still being worked through. Don't block PRs on drift; surface