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.

docs/tools/forge.md

@@ -47,6 +47,9 @@ hive-forge lint stale-branches --days 14    # branches with no recent activity
 hive-forge lint assignments                 # per-assignee open item count
 hive-forge pr-status --pr 42                # PR health: mergeable, CI, reviews, last comment (exit 0 = ready)
 hive-forge pr-status --sha <sha>            # CI-only fast path for an explicit commit sha
+hive-forge pr-merge 42                       # merge (refuses unless mergeable + CI not red + no changes-requested); deletes head branch
+hive-forge pr-merge 42 --method rebase       # rebase-merge instead of a merge commit (no squash option)
+hive-forge pr-merge 42 --keep-branch --force # keep the head branch; override the readiness gate
 hive-forge timeline 42                      # audit trail: closes, label changes, assignments, commit refs
 hive-forge attach-issue 42 /path/to/file    # upload a file attachment to an issue; prints download URL
 hive-forge attach-comment 18042 /path/to/file  # upload a file attachment to a comment; prints download URL