ci: release pull requests, and conventional-commit PR titles

Pull requests only, squash-merged, titled as conventional commits (checked); release-bot keeps a
chore(release) pull request open, worked out by git-cliff, and merging it tags the release.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.gitea/default_merge_message/SQUASH_TEMPLATE.md

@@ -0,0 +1,3 @@
+${PullRequestTitle} (#${PullRequestIndex})
+
+${PullRequestDescription}