feat(cli): menu contextuel d'autocomplétion des slash-commands dans le composer — #163 (QA verte)

À la saisie d'un « / » en début de draft (sans espace), ouverture d'un menu
contextuel listant les commandes matchant le préfixe, via le contrat unifié
listSlashCommands — aucune liste codée en dur côté UI. Rafinement incrémental
des suggestions à la frappe, navigation clavier + sélection, exécution par
executeSlashCommand.

- adapters/agent: listSlashCommands(prefix?) + executeSlashCommand (invoke Tauri).
- domain + ports: types SlashCommand / ExecuteSlashCommandResult.
- mock: données de test.
- CustomAgentChatView: détection de préfixe (/ sans espace), fetch avec séquence
  anti-retard, gestion ouverture/active index, navigation clavier, fermeture
  quand le préfixe cesse d'être éligible.

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

frontend/src/features/agents/CustomAgentChatView.test.tsx

@@ -380,6 +380,148 @@ describe("CustomAgentChatView", () => {
     expect(screen.getByText("Progress").parentElement?.textContent).toContain("done");
   });
 
+  it("shows slash-command suggestions from the gateway, filters by prefix, and inserts the selected command", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi.fn(async () => {}),
+      listSlashCommands: vi.fn(async (prefix?: string) =>
+        [
+          {
+            name: "/clean",
+            shortDescription: "Nettoyer la conversation courante",
+            requiresConfirmation: false,
+            availability: { status: "available" as const },
+            source: "native" as const,
+            native: "clean" as const,
+          },
+          {
+            name: "/commit",
+            shortDescription: "Préparer un commit",
+            requiresConfirmation: true,
+            availability: { status: "available" as const },
+            source: { plugin: { pluginId: "git" } },
+          },
+        ].filter((command) => !prefix || command.name.startsWith(prefix)),
+      ),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="chat-session-1"
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "chat-session-1",
+        expect.any(Function),
+      ),
+    );
+
+    const composer = screen.getByLabelText(/message CLI custom/) as HTMLTextAreaElement;
+    fireEvent.change(composer, { target: { value: "/c" } });
+
+    const menu = await screen.findByRole("listbox", {
+      name: "suggestions commandes slash",
+    });
+    expect(agent.listSlashCommands).toHaveBeenLastCalledWith("/c");
+    expect(screen.getByRole("option", { name: /\/clean/ })).toBeTruthy();
+    expect(screen.getByRole("option", { name: /\/commit/ })).toBeTruthy();
+
+    fireEvent.keyDown(composer, { key: "ArrowDown" });
+    fireEvent.keyDown(composer, { key: "Enter" });
+
+    await waitFor(() => expect(composer.value).toBe("/commit "));
+    expect(menu).toBeTruthy();
+    expect(screen.queryByRole("listbox", { name: "suggestions commandes slash" })).toBeNull();
+    expect(agent.sendAgentChat).not.toHaveBeenCalled();
+  });
+
+  it("only opens slash-command suggestions at the start of an unfinished slash token", async () => {
+    const agent = {
+      launchAgentChat: vi.fn(),
+      reattachAgentChat: vi.fn(async (sessionId: string) => ({
+        sessionId,
+        scrollback: [],
+      })),
+      sendAgentChat: vi.fn(async () => {}),
+      listSlashCommands: vi.fn(async () => [
+        {
+          name: "/clean",
+          shortDescription: "Nettoyer la conversation courante",
+          requiresConfirmation: false,
+          availability: { status: "available" as const },
+          source: "native" as const,
+          native: "clean" as const,
+        },
+      ]),
+      cancelAgentChat: vi.fn(async () => {}),
+      closeAgentChat: vi.fn(async () => {}),
+    };
+
+    render(
+      <DIProvider
+        gateways={{
+          agent,
+          system: { pickFile: vi.fn(async () => null) },
+        } as unknown as Gateways}
+      >
+        <CustomAgentChatView
+          projectId="project-1"
+          agentId="agent-1"
+          agentName="Worker"
+          profile={profile}
+          cwd="/repo"
+          nodeId="node-1"
+          sessionId="chat-session-1"
+          conversationId="conversation-1"
+          onSessionId={vi.fn()}
+          onConversationId={vi.fn()}
+        />
+      </DIProvider>,
+    );
+
+    await waitFor(() =>
+      expect(agent.reattachAgentChat).toHaveBeenCalledWith(
+        "chat-session-1",
+        expect.any(Function),
+      ),
+    );
+
+    const composer = screen.getByLabelText(/message CLI custom/) as HTMLTextAreaElement;
+    fireEvent.change(composer, { target: { value: "hello /c" } });
+    expect(screen.queryByRole("listbox", { name: "suggestions commandes slash" })).toBeNull();
+    expect(agent.listSlashCommands).not.toHaveBeenCalled();
+
+    fireEvent.change(composer, { target: { value: "/" } });
+    await screen.findByRole("listbox", { name: "suggestions commandes slash" });
+
+    fireEvent.change(composer, { target: { value: "/clean now" } });
+    expect(screen.queryByRole("listbox", { name: "suggestions commandes slash" })).toBeNull();
+  });
+
   it("pastes a clipboard image as a removable preview chip", async () => {
     const agent = {
       launchAgentChat: vi.fn(),