ci(release): write the release page from a template, with an English summary

[fylorn]

What this changes

Release pages get a title and a fixed layout. A release is titled "ThinkWatch Lite ", and its text is, in order: an English summary from release-notes/<version>.md when that file exists, a table of the file for each platform with the Homebrew and Linux install commands, how to verify a download against its .sha256, and GitHub's generated list of pull requests. Until now the page was only the generated list, under the bare tag.

Why

The release page is where people who download by hand arrive, and it did not say which file is for which machine, how to install from the command line, or how to check a download. The tag message cannot fill that role: it is the Chinese update text that goes into latest.json.

How it works

• scripts/release_notes.py <version> <generated list> writes the text. It refuses a malformed version, and a summary that is empty, contains Chinese characters or starts a top-level heading (the workflow sets the title).
• New job release-body (ubuntu, runs on tags and on rehearse/**): asks releases/generate-notes for the list, runs the script, writes the result to the run summary and uploads it as an artifact. publish now needs it.
• publish passes name and body_path to action-gh-release and no longer sets generate_release_notes. It writes the text only when the release does not exist yet (the same rule as ThinkWatch-Core#191), so a re-run neither appends a second list nor overwrites a page edited after publishing.
• The comment on the tag-notes step is corrected: since 2026.9.13 (fix(update): announce a new version with a notification, and show only the version #153) the update window shows only the version; the tag text still goes into latest.json, and older installs show it.
• CONTRIBUTING gets a Releases section: the tag, the rehearsal branch, and the two texts.
• release-notes/2026.9.16.md is the English version of the 2026.9.16 tag notes.

How it was verified

• python3 scripts/release_notes_test.py: 17 tests, run on Python 3.14 and 3.9. They check the five file names against the manifest step in release.yml, the install and checksum commands, that no winget command appears, the summary checks, and every file in release-notes/. Changing one file name in release.yml, or adding a Chinese line to a summary, fails them.
• actionlint with shellcheck on both workflows: nothing in the new steps (the existing SC2046 in the signing step is untouched). CI runs actionlint without shellcheck, as before.
• Rehearsal: run 36091652370 on rehearse/release-notes (this commit). The release-body job finished in under a minute; its artifact (the same file the job appends to the run summary) is byte-identical to the page rendered locally for 2026.9.16. The five build jobs were cancelled once that was known and then re-run to completion (attempt 2): all passed, and publish was skipped, as in every rehearsal.
• The list from generate-notes for v2026.9.16 is byte-identical to the list the 2026.9.16 page carried before this change, so a real release gets the same list as before.

Notes for review

• Code comments follow the language already used in release.yml and scripts/; everything that reaches the release page is English.
• The live 2026.9.16 release now has this layout and title; its text is the rehearsal artifact. The 2026.9.15 release got a short correction at the end, since its list names Linux packaging (AppImage + deb) and self-update #169 "Linux packaging (AppImage + deb)" while only the AppImage shipped.

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