fix(release): include changelog in tagged commits

[GSTJ]

Summary

Release tags now wait for their generated changelog entry to merge. The tagged commit and GitHub release body contain the same notes.

Details

• Adds an --upcoming mode that generates an entry before its tag exists.
• Makes the release script a two-pass flow: prepare the changelog on current main, merge it through a PR, and rerun the script to create the tag.
• Excludes changelog preparation commits from generated notes, including the existing post-tag changelog commits.
• Refuses to create a tag from a branch other than current main, a dirty checkout, or a checkout behind origin/main.

Testing steps

1. Run:

pnpm test:release

2. Confirm the command exits successfully and the terminal ends with OK.

[GSTJ]

Local proof from 3be8de1: the integration suite covers the pre-tag stop, the post-merge tag, matching changelog output, and breaking-change notes.