refactor(#2441): update queue consumers + docs for agent-per-node

DagView no longer has a DAG-level agent, so consumers derive it from the
per-node agents:
- hivectl dag_progress.rs: dag_agents(d) helper (distinct node agents,
  comma-joined) in place of d.agent.
- dashboard builds.js: entryAgents(entry) helper likewise for the
  rebuild-queue card + live-log header + cancel confirms.
- docs/coordinator.md: lease prose (node-agent-keyed, global per agent),
  wire shape (NodeView.agent, no DagView.agent), and dropped the removed
  dedup section.

frontend/packages/dashboard/src/builds.js

@@ -165,11 +165,22 @@ function firstFailedNode(entry) {
   return (entry.nodes || []).find((n) => n.state === 'failed') || null;
 }
 
+// Distinct agents across a DAG's nodes, comma-joined for display. Agent is
+// per-node now (a DAG can span agents, e.g. a hive-wide restart), so there's
+// no DAG-level `agent` field — derive it from the nodes.
+function entryAgents(entry) {
+  const seen = [];
+  for (const n of entry.nodes || []) {
+    if (n.agent && !seen.includes(n.agent)) seen.push(n.agent);
+  }
+  return seen.join(',');
+}
+
 function rebuildQueueEntryFingerprint(entry, isChild) {
   return JSON.stringify({
     state: entry.state,
     kind: entry.kind,
-    agent: entry.agent,
+    agent: entryAgents(entry),
     source: entry.source,
     started_at: entry.started_at,
     enqueued_at: entry.enqueued_at,
@@ -265,7 +276,7 @@ function renderQueueEntry(entry, _byId, isChild) {
     el('span', { class: 'rqe-kind', title: entry.kind },
       (QUEUE_KIND_GLYPH[entry.kind] || '?') + ' ' + entry.kind),
     ' ',
-    el('code', { class: 'rqe-agent' }, entry.agent),
+    el('code', { class: 'rqe-agent' }, entryAgents(entry)),
   );
   li.append(' ', el('span', { class: 'rqe-source rqe-source-' + entry.source }, entry.source));
   if (entry.state === 'queued') {
@@ -331,14 +342,14 @@ function renderQueueEntry(entry, _byId, isChild) {
       class: 'inline rqe-cancel',
       'data-async': '',
       'data-confirm':
-        `cancel ${entry.kind} for \`${entry.agent}\` (queue id ${entry.id})? ` +
+        `cancel ${entry.kind} for \`${entryAgents(entry)}\` (queue id ${entry.id})? ` +
         `the row drops from the queue and never runs. running / done / failed entries can't be cancelled this way.`,
     });
     cancelForm.append(el('button', {
       type: 'submit',
       class: 'rqe-cancel-btn',
       title: 'cancel this queued ' + entry.kind,
-      'aria-label': 'cancel queued ' + entry.kind + ' for ' + entry.agent,
+      'aria-label': 'cancel queued ' + entry.kind + ' for ' + entryAgents(entry),
     }, '✗'));
     li.append(cancelForm);
   }
@@ -414,7 +425,7 @@ function renderRebuildLiveLog(queue) {
   const header = el('div', { class: 'rebuild-live-log-header' },
     toggle, ' ',
     el('span', { class: 'rebuild-live-log-title' }, 'live build log — '),
-    el('code', { class: 'rqe-agent' }, running.agent),
+    el('code', { class: 'rqe-agent' }, entryAgents(running)),
     ' ', el('span', { class: 'rqe-kind' },
       (running.kind || 'rebuild') + ' · ' + (NODE_KIND_LABEL[liveNode.kind] || liveNode.kind)),
     ' ', badge, ' ',