fix(release): make changelog promote idempotent (no duplicate version headers)

The promote step unconditionally inserted "## [version]" after "## [Unreleased]"
on every build, so repeated or same-version releases produced empty, duplicated
version headers (e.g. two "## [02.56.07]"). Guard the insert on the version
header not already being present.

Local stopgap; the same fix is needed in the template auto-release.yml
(tracked in Template-Generic).

Claude-Session: https://claude.ai/code/session_01WbGBN9VyRK61zczYWcCQ2i

.mokogitea/workflows/auto-release.yml

@@ -380,10 +380,13 @@ jobs:
           import sys
           version, date = sys.argv[1], sys.argv[2]
           content = open('CHANGELOG.md').read()
-          old = '## [Unreleased]'
-          new = f'## [Unreleased]\n\n## [{version}] --- {date}'
-          content = content.replace(old, new, 1)
-          open('CHANGELOG.md', 'w').write(content)
+          marker = f'## [{version}]'
+          # Idempotent: only promote when this version header isn't already present,
+          # otherwise re-runs (or same-version builds) create empty duplicate headers.
+          if marker not in content:
+              new = f'## [Unreleased]\n\n{marker} --- {date}'
+              content = content.replace('## [Unreleased]', new, 1)
+              open('CHANGELOG.md', 'w').write(content)
           " "$VERSION" "$DATE"
             git add CHANGELOG.md
             git commit -m "chore: promote changelog [Unreleased] → [${VERSION}]" || true

CHANGELOG.md

@@ -1,6 +1,9 @@
 # Changelog
 ## [Unreleased]
 
+### Fixed
+- Release automation: the changelog-promote step is now idempotent — it only inserts a `## [version]` header when that version isn't already present, preventing the empty, duplicated version headers that repeated/same-version builds were producing. (workflow)
+
 ## [02.56.08] --- 2026-07-05
 
 ## [02.56.08] --- 2026-07-05