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.

hive-c0re/src/bin/hivectl/dag_progress.rs

@@ -68,7 +68,7 @@ async fn wait_for_dags_plain(socket: &Path, ids: Vec<u64>) -> Result<()> {
                 // sees whether the agent came back.
                 if d.nodes.iter().all(|n| n.state.is_terminal()) {
                     if d.state == hive_sh4re::jobs::State::Failed {
-                        failed.push(format!("{} {}", d.kind.as_str(), d.agent));
+                        failed.push(format!("{} {}", d.kind.as_str(), dag_agents(d)));
                     }
                 } else {
                     all_terminal = false;
@@ -138,7 +138,7 @@ async fn wait_for_dags_animated(socket: &Path, ids: Vec<u64>) -> Result<()> {
                     "{} {} {} · {}",
                     state_glyph(d.state),
                     d.kind.as_str(),
-                    d.agent,
+                    dag_agents(d),
                     fmt_dur(dag_elapsed(d, now)),
                 ));
                 for n in &d.nodes {
@@ -166,7 +166,7 @@ async fn wait_for_dags_animated(socket: &Path, ids: Vec<u64>) -> Result<()> {
                 }
                 if d.nodes.iter().all(|n| n.state.is_terminal()) {
                     if d.state == hive_sh4re::jobs::State::Failed {
-                        failed.push(format!("{} {}", d.kind.as_str(), d.agent));
+                        failed.push(format!("{} {}", d.kind.as_str(), dag_agents(d)));
                     }
                 } else {
                     all_terminal = false;
@@ -204,6 +204,19 @@ fn finish_wait(mut failed: Vec<String>) -> Result<()> {
     }
 }
 
+/// Distinct agents across a DAG's nodes, comma-joined for display — the
+/// per-node replacement for the old DAG-level `agent` field. Single-agent
+/// DAGs render one name; a hive-wide DAG lists each.
+fn dag_agents(d: &hive_sh4re::jobs::DagView) -> String {
+    let mut seen: Vec<&str> = Vec::new();
+    for n in &d.nodes {
+        if !seen.contains(&n.agent.as_str()) {
+            seen.push(&n.agent);
+        }
+    }
+    seen.join(",")
+}
+
 /// Current unix time in seconds (0 on the impossible pre-epoch error).
 fn now_unix() -> i64 {
     std::time::SystemTime::now()
@@ -290,7 +303,7 @@ fn render_dag_line(d: &hive_sh4re::jobs::DagView) -> String {
         "{} {} {:<12}",
         state_glyph(d.state),
         d.kind.as_str(),
-        d.agent
+        dag_agents(d)
     );
     for (i, n) in d.nodes.iter().enumerate() {
         let sep = if i == 0 { "  " } else { " → " };
@@ -314,9 +327,10 @@ mod tests {
 
     use super::render_dag_line;
 
-    fn node(id: u32, kind: &str, state: State, step: Option<&str>) -> NodeView {
+    fn node(id: u32, agent: &str, kind: &str, state: State, step: Option<&str>) -> NodeView {
         NodeView {
             id,
+            agent: agent.to_owned(),
             kind: kind.to_owned(),
             deps: if id == 0 { vec![] } else { vec![id - 1] },
             state,
@@ -332,7 +346,6 @@ mod tests {
     fn render_dag_line_shows_chain_and_running_step() {
         let dag = DagView {
             id: 7,
-            agent: "alice".to_owned(),
             kind: Template::Rebuild,
             state: State::Running,
             source: Source::Manual,
@@ -345,10 +358,16 @@ mod tests {
             approval_id: None,
             perm_payload: None,
             nodes: vec![
-                node(0, "prebuild", State::Done, None),
-                node(1, "stop_for_update", State::Done, None),
-                node(2, "swap", State::Running, Some("nixos-container update")),
-                node(3, "reconcile", State::Queued, None),
+                node(0, "alice", "prebuild", State::Done, None),
+                node(1, "alice", "stop_for_update", State::Done, None),
+                node(
+                    2,
+                    "alice",
+                    "swap",
+                    State::Running,
+                    Some("nixos-container update"),
+                ),
+                node(3, "alice", "reconcile", State::Queued, None),
             ],
         };
         let line = render_dag_line(&dag);
@@ -363,11 +382,10 @@ mod tests {
 
     #[test]
     fn render_dag_line_surfaces_first_node_error() {
-        let mut failed = node(0, "prebuild", State::Failed, None);
+        let mut failed = node(0, "bob", "prebuild", State::Failed, None);
         failed.error = Some("nix build exploded".to_owned());
         let dag = DagView {
             id: 8,
-            agent: "bob".to_owned(),
             kind: Template::Rebuild,
             state: State::Failed,
             source: Source::Manual,