A production deploy's app.serving event carries the release's version and pull titles, read by one tested function #6098

Part of #5807, split out. The reviewer's ask on pull #5831 three times was that the producer is tested.

Done when:

• [ ] src/core/release_subject.ts exports the subject parser (Release 0.7.251: <title> (#N);<title> (#M) gives {version, pulls: [{title, number}]}) and releaseOfCommit(repoDir, env, oid). It reads the commit's subject with commitLog and returns the parsed release for production, and undefined for a preview or a plain subject.
• [ ] AppsRuntime (src/apps/runtime.ts) calls it when a production deploy goes live and adds version and pulls to the app.serving payload. Previews add nothing.
• [ ] test/release-subject.test.ts git inits a repository under $TMPDIR and makes a release commit and a plain one. It asserts the parsed release for production, undefined for the same commit as a preview, and undefined for the plain commit. Deleting the env check fails it.
• [ ] test/release-words.test.ts (the #5658 guard) is unchanged.

The branch of pull #5831 holds most of this.