ci(release): use stock tools for the feed, release and plugin manifest

[datlechin]

Why

The release tooling re-implemented what gh, git, Actions and the Python stdlib already do. Audit items: feed-2 to feed-12, feed-missed-1, feed-missed-2, notes-1, notes-missed-1, pipeline-1, pipeline-4, pipeline-5 (release.yml), pipeline-12, pipeline-13, docs-1, docs-5, docs-6 and docs-12 (feed.py, RELEASING.md Keys and environments, ci.md), docs-14, build-9, plugins-1, plugins-2, plugins-4, plugins-5, plugins-missed-1 (Python, JSON and check parts).

Change

• github-release: actions/attest, then gh release create (draft, upload, publish), then gh release verify and verify-asset. publish.py goes; feed.py latest serves withdraw.yml.
• feed and withdraw: feed.py edits a gh-pages checkout and the job commits and pushes. Concurrency gh-pages with queue: max replaces the retry loop; actionlint 1.7.12 ignores that key until Actionlint does not know about queue: key for concurrency rhysd/actionlint#657 lands. Assets are checked with gh release verify-asset, generate_appcast signs, withdraw edits with xml.dom.minidom, and both refuse an appcast whose signature does not verify.
• Manifest: writes exportFormatIds, drops iconName. registry.py loses its dead CLI and re-checks; check-plugin-manifest.py reads project.yml with PyYAML.
• version.py check refuses a stable tag that still holds fragments.

Verified

scripts/release and scripts/ci tests, actionlint 1.7.12, shellcheck. test_feed.py, test_registry.py and test_check_plugin_manifest.py are rewritten for the new shapes; test_publish.py goes with publish.py.

Not in this PR

• The Swift side of plugins-1 and plugins-2 and its docs PR are lane L3. Both merge before the first tag.
• RELEASING.md conflicts with build(release): build, sign and package with stock Apple tools #3512 on the adjacent verify and app bullets. Whichever merges second merges main in and keeps both: the fragment clause on verify from this PR, the app wording from build(release): build, sign and package with stock Apple tools #3512.