First half of making the startup sweep one DAG. Adds the runtime
subgraph-append machinery and switches the sweep MetaLock from fanning out
child Rebuild DAGs to growing one rebuild subgraph per stale agent into the
same DAG:
- JobQueue::append_subgraph(dag_id, nodes, dep_on) — the multi-node,
multi-agent generalisation of append_node: rebases a subgraph's local deps
onto the DAG's id space and roots it on the emitting node.
- NodeOutput.append_subgraph: Vec<Vec<NodeSpec>> — the executor→scheduler
channel for it; scheduler drains it before completing the emitting node
(same ordering as append_nodes).
- run_meta_lock sweep branch returns the stale agents' rebuild_nodes
subgraphs via append_subgraph instead of fanout.
Follow-up commit collapses submit_boot_tree (drop boot_root + per-agent
reconcile child DAGs) so the whole boot is one DAG built inline.
b6defdeaaf