feat(hive-forge): add pr-merge verb (#1670)

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.

hive-forge/src/verbs/mod.rs

@@ -23,6 +23,7 @@ pub mod list;
 pub mod milestone;
 pub mod pr;
 pub mod pr_create;
+pub mod pr_merge;
 pub mod pr_reviews;
 pub mod pr_status;
 pub mod repo_add_collaborator;