builds: drop per-entry agent grouping from DAG renderer

Removes entryAgents() and the two places it rendered agent names:
- rqe-agent code element in the entry header
- rqe-node-agent-label prefix per component chain when multi-component

The DAG structure split (WCC + fan-out) already communicates subgraph
boundaries visually via the separate .rqe-nodes rows; the agent-name
labels on top of that caused layout breakage (#2666) and duplicate
information. Closes #2666.

The live build log header still labels liveNode.agent (a single specific
node, not the whole DAG) — that .rqe-agent rule is kept.

Also removes the now-unused .rqe-node-agent-label CSS rule and its
comment.

frontend/packages/dashboard/src/system-sections.css

@@ -159,10 +159,7 @@
 .rqe-node-cancelled { opacity: 0.55; text-decoration: line-through; }
 .rqe-node-arrow { color: var(--muted); }
 .rqe-node-log { margin-left: 0.1em; text-decoration: none; }
-/* Per-agent subgraph label, shown only when a DAG's nodes span more than
-   one agent (independent concurrent subgraphs, one `.rqe-nodes` line each —
-   see renderQueueEntry in builds.js). */
-.rqe-node-agent-label { color: var(--amber); margin-right: 0.3em; }
+
 .rqe-step {
   flex-basis: 100%;
   margin: 0.1em 0 0 1.8em;