ci: seed changelog entry on regen version bump

Author: zfarrell

.github/workflows/regenerate.yml

@@ -75,6 +75,39 @@ jobs:
             | jq -r '.packages[] | select(.name=="hotdata") | .version')
           echo "version=$version" >> "$GITHUB_OUTPUT"
 
+      # check-release.yml gates merges on a `## [x.y.z]` CHANGELOG section
+      # matching the bumped version, so without a seeded entry every regen PR
+      # fails that check. Insert a stub (the spec-change title under ### Changed)
+      # right after `## [Unreleased]`; the PR author refines the wording before
+      # merge. Idempotent: skips if a section for this version already exists.
+      - name: Seed changelog entry
+        env:
+          VERSION: ${{ steps.pkg.outputs.version }}
+          TITLE: ${{ inputs.title }}
+        run: |
+          export CHANGELOG_DATE=$(date -u +%Y-%m-%d)
+          python3 - <<'PY'
+          import os, pathlib, re
+          version = os.environ["VERSION"]
+          date = os.environ["CHANGELOG_DATE"]
+          title = (os.environ.get("TITLE") or "").strip() \
+              or "Regenerate the client from the updated Hotdata OpenAPI spec"
+          path = pathlib.Path("CHANGELOG.md")
+          text = path.read_text()
+          if re.search(rf"^## \[{re.escape(version)}\]", text, re.M):
+              print(f"CHANGELOG already has a [{version}] section; leaving it untouched.")
+              raise SystemExit(0)
+          marker = "## [Unreleased]\n"
+          idx = text.find(marker)
+          if idx == -1:
+              raise SystemExit("CHANGELOG.md has no '## [Unreleased]' section to anchor the new entry")
+          insert_at = idx + len(marker)
+          entry = f"## [{version}] - {date}\n\n### Changed\n\n- {title}\n"
+          text = text[:insert_at] + "\n" + entry + "\n" + text[insert_at:].lstrip("\n")
+          path.write_text(text)
+          print(f"Inserted CHANGELOG [{version}] section.")
+          PY
+
       - name: Generate client
         env:
           PACKAGE_VERSION: ${{ steps.pkg.outputs.version }}