fix(plugins): isole les menus plugin et durcit hello-plugin SDK contre la rechute #120

Requalification Architect du 2026-08-01: le crash INTERFACE INTERROMPUE au
chargement d'un plugin (menu + item) remonte via ProjectsView -> usePluginMenus
-> MenuBar jusqu'à RootErrorBoundary, quel que soit le plugin (ancien ou
reconstruit via SDK).

- usePluginMenus isole la résolution/conversion des contributions plugin :
  toute erreur retombe sur [] au lieu de propager.
- ProjectsView sépare les menus natifs des menus enrichis par plugin et rend
  MenuBar derrière une error boundary locale (fallback menus natifs seuls).
- hello-plugin (SDK) et les tests d'installation associés durcis en cohérence.

Bookkeeping ticket #120 uniquement (issue.md, carnet.md) ; les fichiers
counter.json/index.json et le dossier tickets/122/ restent hors commit car
une collision de numérotation #122 existe entre cette base et
feature/ticket120-hello-plugin-install-path-audit (deux tickets différents
revendiquent #122) — à arbitrer avant de committer le bookkeeping global.

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

frontend/src/features/plugins/usePluginMenus.test.tsx

@@ -0,0 +1,75 @@
+import { renderHook } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+
+import type { MenuTargetId } from "@/domain";
+import type { PluginRuntimeRegistry, WhenContext } from "@/plugins/runtime";
+import { usePluginMenus } from "./usePluginMenus";
+
+const NO_CONTEXT: WhenContext = {
+  projectOpen: false,
+  gitRepository: false,
+  agentSelected: false,
+  terminalFocused: false,
+  layoutCellFocused: false,
+};
+
+function throwingRegistry(overrides: Record<string, unknown>): PluginRuntimeRegistry {
+  return {
+    runCommand: vi.fn(async () => {}),
+    topLevelMenus: vi.fn(() => []),
+    menuItems: vi.fn(() => []),
+    ...overrides,
+  } as unknown as PluginRuntimeRegistry;
+}
+
+describe("usePluginMenus", () => {
+  it("falls back to no plugin top-level menus when plugin menu resolution throws", () => {
+    const consoleWarn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      const registry = throwingRegistry({
+        topLevelMenus: vi.fn(() => {
+          throw new Error("broken top-level contribution");
+        }),
+      });
+
+      const { result } = renderHook(() => usePluginMenus(registry, NO_CONTEXT));
+
+      expect(result.current.topLevelMenus).toEqual([]);
+      expect(consoleWarn).toHaveBeenCalledWith(
+        "[plugins] menu contribution rejected",
+        expect.objectContaining({
+          scope: "top-level-menus",
+          reason: "broken top-level contribution",
+        }),
+      );
+    } finally {
+      consoleWarn.mockRestore();
+    }
+  });
+
+  it("falls back to no plugin items for a native menu when item resolution throws", () => {
+    const consoleWarn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      const registry = throwingRegistry({
+        menuItems: vi.fn(() => {
+          throw new Error("broken item contribution");
+        }),
+      });
+
+      const { result } = renderHook(() => usePluginMenus(registry, NO_CONTEXT));
+      const items = result.current.itemsFor("panels" as MenuTargetId);
+
+      expect(items).toEqual([]);
+      expect(consoleWarn).toHaveBeenCalledWith(
+        "[plugins] menu contribution rejected",
+        expect.objectContaining({
+          scope: "items",
+          targetMenuId: "panels",
+          reason: "broken item contribution",
+        }),
+      );
+    } finally {
+      consoleWarn.mockRestore();
+    }
+  });
+});