fix(layout): ré-attacher le flux d'une cellule au réveil arrière-plan d'un agent

Une cellule ne ré-attachait pas le flux de sortie de son agent lorsque
celui-ci était réveillé en arrière-plan par une délégation : l'utilisateur
devait basculer la vue Plain↔agent pour voir l'agent travailler.

Ajout d'un effect de ré-attache déclenché sur le réveil arrière-plan + bump
de la key pour forcer le remontage de la vue. Test de régression
liveReattachOnDelegation : rouge sans le fix, vert avec.

Vérifs : tsc --noEmit exit 0 ; vitest run 48 fichiers / 444 tests OK.

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

frontend/src/features/layout/LayoutGrid.tsx

@@ -329,6 +329,47 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
   const [pendingResume, setPendingResume] = useState<PendingResume | null>(null);
   const [resumeDetails, setResumeDetails] = useState<ConversationDetails | null>(null);
 
+  // ── Live re-attach on background wake (inter-agent delegation) ─────────────
+  // Bumped to re-key the TerminalView and force a re-attach to the cell's
+  // pinned agent when that agent becomes live in the *background* AFTER this
+  // cell has mounted — the classic case being an inter-agent delegation: the
+  // orchestrator wakes a cold target with `node_id = None` (background) and
+  // publishes `AgentLaunched`. Without this, the cell keeps showing its
+  // previous (often dead) session and only re-subscribes to the live output
+  // once the user toggles the view (Plain↔agent), which remounts the terminal.
+  // We do that re-attach declaratively instead, so the delegated turn is
+  // visible live with no manual toggle. The counter is bumped ONLY here (the
+  // background-wake path), never on a normal fresh launch — so ordinary mounts
+  // are untouched and never thrash.
+  const [attachGen, setAttachGen] = useState(0);
+  useEffect(() => {
+    if (!agentId || !agentGateway?.attachLiveAgent) return;
+    // Mid resume decision: let the popup own the (re)launch — don't race it.
+    if (pendingResume) return;
+    // Only act when the pinned agent is live in ANOTHER (non-visible) cell and
+    // this cell is not already bound to that session. `backgroundLive` is the
+    // same guard `doLaunch` and the dropdown use; crucially it ignores a session
+    // hosted by THIS cell (nodeId === id), so a fresh self-launch never triggers
+    // a re-attach loop.
+    const bg = backgroundLive(agentId);
+    if (!bg?.sessionId || bg.sessionId === session) return;
+    let cancelled = false;
+    void (async () => {
+      const attached = await agentGateway.attachLiveAgent!(projectId, agentId, id);
+      if (cancelled) return;
+      await vm.attachLiveAgentToCell(id, agentId, attached.sessionId ?? bg.sessionId);
+      if (!cancelled) setAttachGen((g) => g + 1);
+    })().catch(() => {
+      /* ignore — the view toggle remains the manual fallback */
+    });
+    return () => {
+      cancelled = true;
+    };
+    // `liveAgents` is the trigger (refreshed by the agentLaunched event below);
+    // `session` guards against re-firing once attached.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [liveAgents, agentId, session]);
+
   // ── Cell routing (Option 1 — Terminal + MCP) ──────────────────────────────
   // Every agent cell renders the raw {@link TerminalView}: the human view is the
   // native interactive PTY (live reasoning + Échap are native CLI behaviours,
@@ -571,7 +612,7 @@ function LeafView({ id, session, agent, conversationId, agentWasRunning, cwd, vm
         }}
       >
         <TerminalView
-          key={`${id}-${agentId ?? "plain"}`}
+          key={`${id}-${agentId ?? "plain"}-${attachGen}`}
           cwd={cwd}
           open={terminalOpener}
           reattach={reattachOpener}