fix(release): use a one-shot PR instead of pushing the bump to main directly

[JohnnyVicious]

Summary

Release run 24308400583 failed because the previous workflow tried to `git push origin HEAD:main` and was rejected by the branch protection rule:

```
remote: error: GH013: Repository rule violations found for refs/heads/main.
remote: - Changes must be made through a pull request.
```

The bump commit and tag never reached origin (the worker's filesystem is gone), so there is nothing to clean up — `origin/main` is still at `f19fbf0` and no `v1.0.1` tag exists.

Fix

Replace the single "commit, tag, and push" step with a PR-mediated flow that satisfies the branch protection rule:

1. Push the bump on a release branch — checkout `release/v${version}`, commit the bumped manifests, push the branch.
2. Open the release PR and merge it — `gh pr create` against main with a descriptive title/body, then `gh pr merge --squash --delete-branch`. The change still reaches main through a PR (which is what the rule wants), while the workflow stays fully automated.
3. Tag the merged commit on main — fetch + `reset --hard origin/main` to pick up the squash-merge commit, tag it, push the tag.
4. Create GitHub release — unchanged, still uses `gh release create --generate-notes`.

Permissions

Added `pull-requests: write` so the workflow can open and merge the release PR. `contents: write` is still needed for the tag push.

Test plan

• YAML well-formed (`python3 -c "import yaml; yaml.safe_load(open(...))"`)
• CI green on this PR (no real workflow execution — release.yml only fires on `workflow_dispatch`)
• End-to-end smoke after merge: re-trigger release with `version: 1.0.1` and verify the release branch is created, the PR opens + merges, the tag lands, and the GitHub release is published

Alternative considered

I considered configuring the branch protection ruleset to add `github-actions[bot]` as a bypass actor instead, which would let the original direct-push flow work as-is. I went with the PR-based flow because it doesn't depend on hidden repo configuration the user has to remember, and it leaves a paper trail in the PR list for every release. Happy to switch if you'd rather have the simpler workflow + bypass list.