fix(agents): enforcer l'invariant « 1 session vivante par agent » (singleton)

Un agent ne peut tourner que dans une seule cellule à la fois. La garde dans
LaunchAgent refuse le spawn si l'agent est déjà vivant dans un autre node
(AGENT_ALREADY_RUNNING) ; idempotent sur le même node ; le chemin resume
(agent mort) reste inchangé. Le node_id est désormais plombé jusqu'au use case.

Corrige le reset asymétrique d'une cellule au changement d'onglet : deux leaves
partageant le même agent id rendaient session_for_agent/is_agent_live/stop_agent
ambigus (cible arbitraire). Le churn reset/reattach déclenchait aussi les accents
mélangés (FIFO intact, non touché).

- snapshot agentWasRunning calculé par node (is_node_live) et non par agent
- commande list_live_agents + live_agents()/node_for_agent()/is_node_live()
- UI : dropdown grise les agents déjà placés ailleurs ; 2e cellule en doublon
  affiche « disponible » au lieu d'une relance fantôme

Tests : cargo test (application + app-tauri) vert ; tsc + vitest vert.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -22,6 +22,7 @@ import type { Agent } from "@/domain";
 import type { LayoutNode } from "@/domain";
 import type {
   ConversationDetails,
+  LiveAgent,
   OpenTerminalOptions,
   TerminalHandle,
 } from "@/ports";
@@ -143,7 +144,7 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
   // the wrong terminal. The root cell (no parent split) cannot be closed.
   const canClose = parentSplit !== null && parentSplit.siblings === 2;
   const siblingIndex = parentSplit ? (parentSplit.index === 0 ? 1 : 0) : 0;
-  const { agent: agentGateway } = useGateways();
+  const { agent: agentGateway, system } = useGateways();
 
   // Load the project's agents for the dropdown.
   const [agents, setAgents] = useState<Agent[]>([]);
@@ -156,9 +157,65 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
     return () => { cancelled = true; };
   }, [agentGateway, projectId]);
 
+  // Load the agents currently running (and where), so the dropdown can disable an
+  // agent already live in another cell — it cannot run in two cells at once. The
+  // backend refuses such a launch (`AGENT_ALREADY_RUNNING`); disabling it here is
+  // the matching UI affordance.
+  const [liveAgents, setLiveAgents] = useState<LiveAgent[]>([]);
+  const refreshLive = () => {
+    if (!agentGateway?.listLiveAgents) return;
+    agentGateway.listLiveAgents(projectId).then(setLiveAgents).catch(() => {
+      /* ignore — treat as none live */
+    });
+  };
+  useEffect(() => {
+    if (!agentGateway?.listLiveAgents) return;
+    let cancelled = false;
+    agentGateway.listLiveAgents(projectId).then((list) => {
+      if (!cancelled) setLiveAgents(list);
+    }).catch(() => {/* ignore */});
+    return () => { cancelled = true; };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [agentGateway, projectId]);
+
+  // Live refresh on agent lifecycle events: a launch/exit in any cell changes
+  // who is running where, so re-pull the live set to keep every dropdown in sync.
+  useEffect(() => {
+    if (!system) return;
+    let unsubscribe: (() => void) | undefined;
+    let cancelled = false;
+    void system
+      .onDomainEvent((event) => {
+        if (event.type === "agentLaunched" || event.type === "agentExited") {
+          refreshLive();
+        }
+      })
+      .then((un) => {
+        if (cancelled) un();
+        else unsubscribe = un;
+      });
+    return () => {
+      cancelled = true;
+      unsubscribe?.();
+    };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [system, agentGateway, projectId]);
+
   // Build the terminal opener based on whether an agent is pinned.
   const agentId = agent ?? null;
 
+  /**
+   * Whether `candidate` is currently running in a cell *other than this one*.
+   * Such an agent cannot be launched here (one live session per agent), so the
+   * dropdown disables it and `onChange` rejects selecting it.
+   */
+  const isLiveElsewhere = (candidate: string): boolean =>
+    liveAgents.some((la) => la.agentId === candidate && la.nodeId !== id);
+
+  // A transient notice shown when an action is blocked by the singleton
+  // invariant (selecting / launching an agent already live in another cell).
+  const [busyNotice, setBusyNotice] = useState<string | null>(null);
+
   // ── Resume popup state (T7) ───────────────────────────────────────────────
   // When an agent cell carries a persisted conversation id and its PTY session
   // is dead, the opener is about to relaunch in Resume mode. We intercept that
@@ -177,7 +234,7 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
     const handle = await agentGateway!.launchAgent(
       projectId,
       agentId!,
-      { ...opts, conversationId: convId },
+      { ...opts, conversationId: convId, nodeId: id },
       onData,
     );
     // First launch on a fresh cell mints a conversation id: persist it on the
@@ -271,6 +328,14 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
           value={agentId ?? ""}
           onChange={(e) => {
             const val = e.target.value;
+            // Reject pinning an agent that is already running in another cell.
+            // (The dropdown also disables such options, but guard the change in
+            // case it is set programmatically.)
+            if (val !== "" && isLiveElsewhere(val)) {
+              setBusyNotice("Cet agent tourne déjà dans une autre cellule.");
+              return;
+            }
+            setBusyNotice(null);
             void vm.setCellAgent(id, val === "" ? null : val);
           }}
           style={{
@@ -284,11 +349,17 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
           }}
         >
           <option value="">Plain</option>
-          {agents.map((a) => (
-            <option key={a.id} value={a.id}>
-              {a.name}
-            </option>
-          ))}
+          {agents.map((a) => {
+            // An agent already running in another cell cannot be pinned here.
+            // The agent pinned on THIS cell stays selectable (same node).
+            const elsewhere = isLiveElsewhere(a.id);
+            return (
+              <option key={a.id} value={a.id} disabled={elsewhere}>
+                {a.name}
+                {elsewhere ? " (en cours ailleurs)" : ""}
+              </option>
+            );
+          })}
         </select>
 
         <button
@@ -329,6 +400,26 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
         sessionId={session}
         onSessionId={(sid) => void vm.setSession(id, sid)}
       />
+      {busyNotice && (
+        <p
+          role="status"
+          style={{
+            position: "absolute",
+            bottom: 4,
+            left: 4,
+            right: 4,
+            margin: 0,
+            fontSize: 11,
+            color: "var(--color-content-muted, #9a9a9a)",
+            background: "var(--color-surface, #1e1e1e)",
+            padding: "2px 6px",
+            borderRadius: 3,
+            zIndex: 3,
+          }}
+        >
+          {busyNotice}
+        </p>
+      )}
       {pendingResume && (
         <ResumeConversationPopup
           agentWasRunning={agentWasRunning}