feat(ci): preview the next release version on pull requests

[raederan]

What's Changed

Conventional commit types decide the version, and nobody saw the consequence
of getting one wrong until after the merge. Every pull request now gets a
sticky comment saying which version a merge would release, or that no bump
happens, with a changelog preview.

Ported from oold-python rather than rewritten, so the details that make it
correct are kept: PSR only computes on main, so the PR head is renamed; full
history is fetched for tag walking; and the changelog stamp runs with every
flag that prevents an actual release.

Checklist

• make ci passes locally (runs check, test and docs-test in one go)
• Commits follow Conventional Commits (the commit-msg hook enforces this)
• Docs updated where behaviour changed

[github-actions]

Release preview

Merging this PR would release v0.11.0 (current: v0.10.0).

Changelog preview (truncated)

## v0.11.0 (2026-08-11)

_{Preview via python-semantic-release and conventional commits.}