Serially land a batch of green PRs through the forge's own merge API — migration-aware ordering, update-branch for stale PRs, wait for the post-merge workflows between merges, then run /worktree-cleanup. Invoke as `/merge-train` (discover all green PRs) or `/merge-train <PR#> [<PR#>…]` (explicit batch). Operator-invoked only; complements /pr-iterate, which never merges.
