Adds `hive-forge pr-merge <n> [--method merge|rebase] [--keep-branch]
[--force]` wrapping POST /repos/{owner}/{repo}/pulls/{n}/merge, so agents on
the peer-review-and-merge workflow have a CLI path instead of the raw API.
- Methods: merge (default) | rebase. Squash is intentionally not offered.
- Deletes the head branch after merge unless --keep-branch.
- Safe by default: refuses unless the PR is mergeable, CI is not red, and no
review's current verdict requests changes (latest-per-reviewer wins, so a
later approval clears an earlier request-changes). --force overrides and
also sets Forgejo's force_merge.
- New client helper post_no_content for the 200-empty-body merge response.
