A release post in the feed reads `<version> is serving: <first pull> and N more`, linking the release commit #6099

Last part of #5807.

Done when:

• [ ] src/feed/items.ts builds the app.serving post from the payload's version and pulls: 0.7.251 is serving: <first title> and N more, linking /<owner>/<repo>/commit/<oid>. With no valid version, the post keeps "a new version is serving".
• [ ] Titles go through the existing escaping, and the version is validated on read.
• [ ] test/home-feed.test.ts covers the words, the link, and the fallback on / and in MCP feed, as a test of how the feed uses the payload.