feat: flag PRs that only join an already-pending release

[non7top]

Fixes #22. For a PR that adds nothing releasable while a release is already pending on the base, the preview now says so up front (A release is already pending on master (vX, a Y bump), and this PR is not what causes it), keeps the pending changelog in a collapsed section, applies a separate RELEASE PENDING label (input pending-label) instead of RELEASE, and sets a new pr-contributes output to false. It works by dry-running release-please on the base branch too and comparing changelogs.

Tested under act against a scratch repo with a pending feat on master: chore PR gives the banner and fold, feat PR gives pr-contributes=true. Label handling is untested (needs a real PR).

[github-actions]

🤖 release-please PREVIEW

What merging this PR into master would trigger:

🤖 I will create a release beep boop

1.6.0 (2026-10-07)

Features

• flag PRs that only join an already-pending release (the merge commit GitHub would create for this PR)
• flag PRs that only join an already-pending release (f8e1cab)
• keep the pending release visible, in a fold, with its own label (77bab33)

---

This PR was generated with Release Please. See documentation.

_{Two entries, one change: release-please reads both the commit on this branch and the merge commit GitHub will create, whose body is this PR title. Both really do land in the published changelog. Only the merge commit has no SHA to cite, because it does not exist yet.}

_{Predicted from 77bab33 merged into master at cf2e67f, with the PR title read as feat: flag PRs that only join an already-pending release · Full run}