fix(chat): stabilise le mode CLI custom contre le refetch stale agents/profiles (#149)

Le fallback custom→TUI se redéclenchait après le premier chargement dès que
refreshProfiles() renvoyait un instant un profil pinné sans structuredAdapter
(refetch en vol), coupant la CLI custom ~0.5s après son ouverture. Introduit
un binding "trusted" (agent+profil validés) qui reste valide tant qu'aucun
changement légitime de profil n'est observé via agentProfileChanged, et ne
retombe en TUI que sur perte réelle du CLI custom.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.chat.test.tsx

@@ -54,7 +54,7 @@ import {
 } from "@/adapters/mock";
 import { DIProvider } from "@/app/di";
 import { leaves } from "./layout";
-import { LayoutGrid } from "./LayoutGrid";
+import { LayoutGrid, shouldFallbackCustomCliMode } from "./LayoutGrid";
 
 const structuredProfile: AgentProfile = {
   id: "mock-structured",
@@ -366,4 +366,66 @@ describe("LayoutGrid custom agent CLI (#147)", () => {
     expect(screen.getByTestId("terminal-view")).toBeTruthy();
     expect(screen.queryByTestId("custom-agent-chat-view")).toBeNull();
   });
+
+  it("does not downgrade a validated custom CLI on a contradictory refetch without a profile-change event (#149)", () => {
+    expect(
+      shouldFallbackCustomCliMode({
+        agentsLoaded: true,
+        profilesLoaded: true,
+        cellMode: "custom",
+        hasPinnedAgent: true,
+        hasPinnedProfile: true,
+        customCliAvailable: false,
+        effectiveCustomCliAvailable: true,
+        hasTrustedCustomCli: true,
+        hasLegitProfileChange: false,
+      }),
+    ).toBe(false);
+  });
+
+  it("falls back to native TUI after a legitimate profile-change event downgrades the profile (#149)", async () => {
+    const layout = new MockLayoutGateway();
+    const agent = new MockAgentGateway();
+    const profileGateway = new MockProfileGateway();
+    const terminal = new MockTerminalGateway();
+    const system = new MockSystemGateway();
+    await profileGateway.configureProfiles([structuredProfile, ptyProfile]);
+    const created = await agent.createAgent("p1", {
+      name: "Worker",
+      profileId: structuredProfile.id,
+    });
+    const tree = await layout.loadLayout("p1");
+    const leafId = leaves(tree)[0].id;
+    await layout.mutateLayout("p1", {
+      type: "setCellAgent",
+      target: leafId,
+      agent: created.id,
+    });
+    window.localStorage.setItem(`idea.agent-cell-mode.p1.${leafId}`, "custom");
+
+    renderGrid({
+      layout,
+      agent,
+      profile: profileGateway,
+      terminal,
+      system,
+    } as unknown as Gateways);
+
+    await waitFor(() =>
+      expect(screen.getByTestId("custom-agent-chat-view")).toBeTruthy(),
+    );
+
+    await agent.changeAgentProfile("p1", created.id, ptyProfile.id, 24, 80);
+    system.emit({
+      type: "agentProfileChanged",
+      agentId: created.id,
+      profileId: ptyProfile.id,
+    });
+
+    await waitFor(() =>
+      expect(window.localStorage.getItem(`idea.agent-cell-mode.p1.${leafId}`)).toBe("tui"),
+    );
+    expect(screen.getByTestId("terminal-view")).toBeTruthy();
+    expect(screen.queryByTestId("custom-agent-chat-view")).toBeNull();
+  });
 });