feat(inter-agent): surface frontend des annonces live sur les cellules (F1-F3)

Affiche les annonces d'agent en direct au-dessus des cellules, sur la base
develop :

- announcements: nouveau feature module — store réactif, provider
  d'abonnement aux événements, overlay par cible et aperçu, avec tests
  (announcementsStore + provider).
- App.tsx: montage du provider dans l'arbre applicatif.
- domain/index.ts: types partagés de l'événement d'annonce côté front.
- layout: composition de l'overlay dans LayoutGrid + règle d'exclusion
  couverte par overlayExclusion.test.ts.
- AgentsPanel: intègre l'aperçu des annonces dans la surface existante.

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

frontend/src/features/layout/LayoutGrid.tsx

@@ -34,9 +34,37 @@ import {
 } from "@/features/terminals";
 import { useGateways } from "@/app/di";
 import { useProjectWorkState } from "@/features/workstate/useProjectWorkState";
+import {
+  TargetAnnouncementsOverlay,
+  useTargetAnnouncements,
+} from "@/features/announcements";
 import { leaves, normalizeWeights, resizeAdjacent } from "./layout";
 import { useLayout, type LayoutViewModel } from "./useLayout";
 
+/**
+ * Full-cell veil exclusion (ticket #4, Architect arbitrage): the write-portal
+ * veil (delegation injection into the PTY) and the F3 target overlay (another
+ * agent is talking to this one) share the same relative container and both cover
+ * `inset:0`. On the inter-agent injection path the write-portal `overlay` flag and
+ * the target's `busy` state rise *together*, which would stack two veils with
+ * near-duplicate banners.
+ *
+ * Rule: exactly one veil at a time, F3 has priority. So the write-portal veil is
+ * shown only for a pinned agent, when the portal asks for it, **and** the target
+ * is not busy-active (F3 owns the cell then). When busy is not yet set — the brief
+ * window before the mediator marks it — the write-portal veil is the fallback.
+ *
+ * Purely visual: this gates rendering only. The keystroke suspension during
+ * injection (`portal.isSuspended()` in `TerminalView`) is untouched.
+ */
+export function shouldShowWritePortalVeil(
+  agentPinned: boolean,
+  writePortalOverlay: boolean,
+  busyActive: boolean,
+): boolean {
+  return agentPinned && writePortalOverlay && !busyActive;
+}
+
 interface LayoutGridProps {
   /** Project whose layout to render. */
   projectId: string;
@@ -313,6 +341,10 @@ function LeafView({
 
   // Build the terminal opener based on whether an agent is pinned.
   const agentId = agent ?? null;
+  // F3 lifecycle state (busy) for this cell's agent — shared here so the
+  // write-portal veil can be mutually excluded with the F3 overlay (exactly one
+  // full-cell veil at a time, F3 first). Same source the overlay itself reads.
+  const { active: busyActive } = useTargetAnnouncements(agentId ?? "");
   const agentWork = agentId
     ? workState?.agents.find((row) => row.agentId === agentId)
     : undefined;
@@ -836,8 +868,9 @@ function LeafView({
         />
         {/* Write-portal overlay (ARCHITECTURE §20.3 step b/e): while a delegation
             is being injected into the agent's PTY, a grey veil with a centred
-            message sits above the terminal. Only ever shown for an agent cell. */}
-        {agentId != null && overlay && (
+            message sits above the terminal. Only ever shown for an agent cell —
+            and never together with the F3 overlay (exactly one veil, F3 first). */}
+        {shouldShowWritePortalVeil(agentId != null, Boolean(overlay), busyActive) && (
           <div
             data-testid="write-portal-overlay"
             role="status"
@@ -868,6 +901,14 @@ function LeafView({
             </span>
           </div>
         )}
+        {/* F3 (ticket #4): while another agent is talking to THIS agent (target),
+            an availability veil showing its live réflexions. Driven by the target's
+            busy state (agentBusyChanged + read-model hydration), never by the raw
+            PTY — it retracts at idle even when a turn ends without a completion
+            event. Self-guards on `active` (busy) and renders null otherwise. */}
+        {agentId != null && (
+          <TargetAnnouncementsOverlay projectId={projectId} agentId={agentId} />
+        )}
       </div>
       {busyNotice && (
         <div