Agent was a single field on Dag/DagSpec, making a DAG structurally
one-agent — a multi-agent op could only ever be N separate DAGs. Move it
onto Node/NodeSpec (and the NodeView wire type), drop it from Dag/DagSpec
(and DagView): a DAG can now span agents.
- lifecycle lease keys on the node's agent, still globally exclusive per
agent across all DAGs (Inner.leases unchanged in shape). A DAG holds one
lease per distinct agent it touches; settle() frees each at DAG-terminal
(per-agent-subgraph early release is a follow-up, only observable with
multi-agent DAGs).
- transient guard keyed (dag_id, agent); cancel-revert + Rebuilt events
walk TerminalDag.agents.
- submit-time dedup removed (a multi-agent DAG has no single agent to key
on); every submit enqueues a fresh DAG. Whether dedup needs reintroducing
is tracked in a follow-up sub-issue.
- templates gain a node(agent, kind, deps) helper stamping the agent onto
every node; meta templates stamp "hyperhive".
Templates stay single-agent in this PR — behaviour is unchanged, only the
representation + wire shape. Multi-agent DAG emission (restart/restart-all/
broad stop+start as one DAG) and the SetWanted-as-a-node change are
follow-ups off #2439.
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