renderQueueEntry flattened entry.nodes into one arrow-joined chain
regardless of which agent each node belongs to. A multi-agent DAG
(e.g. hivectl restart --graceful with several agents) runs independent
per-agent subgraphs concurrently, no cross-agent deps, so joining them
all into one sequential-looking chain misrepresented the actual DAG
shape (mara's report: 'expected one dag that forks after the start
node into the per agent sub dags').
Group nodes by n.agent (stable, first-seen order) and render one
.rqe-nodes line per agent, with a small agent-label chip when the DAG
spans more than one. Single-agent DAGs (the common case) collapse back
to exactly the prior one-line render — no visible change there.
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