chore: clean, fold, and sync the release changelog PR

Expands the release-cut changelog curation so the release PR looks good
by default. curate_changelog.py now normalizes the newest section
(unescape HTML entities, de-link stray @mentions, drop duplicate
entries from the same change, lowercase each entry's leading word),
folds large releases under a <details> block (sized by
--fold-threshold / CHANGELOG_FOLD_THRESHOLD, default 12), and can emit
the curated section via --section-out. The release-cut workflow now
mirrors that section into the PR description, and its curation steps run
whenever a release PR exists (not only when newly created), so
regenerate runs stay curated too.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 944860270

.github/workflows/release-cut.yml

@@ -110,22 +110,24 @@ jobs:
           manifest-file: ${{ steps.config.outputs.manifest_file }}
           target-branch: ${{ steps.config.outputs.candidate_branch }}
 
-      # Curate the changelog: draft a Highlights section on top of the
-      # release-please output and commit it back to the release PR branch. The
+      # Curate the changelog: clean up and (for large releases) fold the
+      # release-please output, draft a Highlights section on top, commit it back
+      # to the release PR branch, and sync the PR description to match. The
       # script falls back to an empty Highlights template if drafting fails, so
-      # this step never blocks the release.
+      # this step never blocks the release. Guarded on `pr` (not `prs_created`)
+      # so it also runs when release-please updates an existing PR (regenerate).
       - name: Set up Python
-        if: steps.release_please.outputs.prs_created == 'true'
+        if: steps.release_please.outputs.pr != ''
         uses: actions/setup-python@v6
         with:
           python-version: '3.11'
 
       - name: Install changelog curation dependencies
-        if: steps.release_please.outputs.prs_created == 'true'
+        if: steps.release_please.outputs.pr != ''
         run: pip install --upgrade google-genai
 
       - name: Curate changelog Highlights
-        if: steps.release_please.outputs.prs_created == 'true'
+        if: steps.release_please.outputs.pr != ''
         # Curation is a nice-to-have layered on top of the release PR; never let
         # it turn the release run red (e.g. a push race or a transient error).
         continue-on-error: true
@@ -137,12 +139,20 @@ jobs:
         run: |
           set -euo pipefail
           PR_BRANCH=$(echo "$RELEASE_PR" | jq -r '.headBranchName')
-          echo "Curating changelog on release PR branch: $PR_BRANCH"
+          PR_NUMBER=$(echo "$RELEASE_PR" | jq -r '.number')
+          echo "Curating changelog on release PR #$PR_NUMBER (branch: $PR_BRANCH)"
           git fetch origin "$PR_BRANCH"
           git checkout -B "$PR_BRANCH" FETCH_HEAD
-          python scripts/curate_changelog.py --changelog CHANGELOG.md
+          python scripts/curate_changelog.py --changelog CHANGELOG.md \
+            --section-out /tmp/pr_body.md
+          # Mirror the curated notes into the PR description so reviewers read
+          # the same thing that ships in CHANGELOG.md. Done regardless of whether
+          # the file changed, since the body is regenerated by release-please.
+          if [ -s /tmp/pr_body.md ]; then
+            gh pr edit "$PR_NUMBER" --body-file /tmp/pr_body.md
+          fi
           if git diff --quiet -- CHANGELOG.md; then
-            echo "No changelog changes to commit."
+            echo "No changelog file changes to commit."
             exit 0
           fi
           USER_JSON=$(gh api user)