feat(tickets): création d'un assistant IA de ticket (backend + frontend)

Ajoute le chat assistant IA attaché à un ticket (#8).

Backend Rust :
- use cases OpenTicketAssistant/CloseTicketAssistant + tests
- politique d'outils par agent (domain/agent_tool_policy) et policy MCP
- store de contexte assistant + gabarit default_ticket_assistant.md + tests
- events TicketAssistantOpened/Closed
- commandes Tauri open_ticket_chat/close_ticket_chat et câblage state/lib/events

Frontend :
- gateway (ports, adapters ticket + mock, domain)
- hook useTicketAssistant + composant TicketAssistantPanel
- intégration dans TicketDetail

Tests verts : vitest tickets.test.tsx (23), cargo test application::ticket_assistant (1),
infrastructure::assistant_context_store (2).

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

frontend/src/features/tickets/tickets.test.tsx

@@ -10,6 +10,7 @@ import {
 import { DIProvider } from "@/app/di";
 import {
   MockAgentGateway,
+  MockProfileGateway,
   MockSystemGateway,
   MockTicketGateway,
 } from "@/adapters/mock";
@@ -43,6 +44,18 @@ function renderView(
   );
 }
 
+async function seedAssistantProfile(profile: MockProfileGateway) {
+  return profile.saveProfile({
+    id: "qa-assistant",
+    name: "QA Assistant",
+    command: "codex",
+    args: [],
+    contextInjection: { strategy: "conventionFile", target: "AGENTS.md" },
+    detect: null,
+    cwdTemplate: "{projectRoot}",
+  });
+}
+
 describe("MockTicketGateway", () => {
   let system: MockSystemGateway;
   let ticket: MockTicketGateway;
@@ -427,6 +440,66 @@ describe("TicketsView", () => {
       ),
     );
   });
+
+  it("opens, streams, and closes the ticket AI assistant chat (#8)", async () => {
+    const t = await ticket.create(PROJECT_ID, { title: "Assistant target" });
+    const profile = new MockProfileGateway();
+    await seedAssistantProfile(profile);
+    const gateways = { ticket, system, agent, profile } as unknown as Gateways;
+
+    render(
+      <DIProvider gateways={gateways}>
+        <TicketDetail
+          projectId={PROJECT_ID}
+          ticketRef={t.ref}
+          onClose={() => {}}
+          onOpenRef={() => {}}
+        />
+      </DIProvider>,
+    );
+
+    const detail = await screen.findByRole("dialog", { name: `ticket ${t.ref}` });
+    expect(within(detail).getByText("Assistant IA")).toBeTruthy();
+
+    const profileSelect = await within(detail).findByLabelText(
+      "profil de l'assistant",
+    );
+    expect(within(detail).getByText("Ouvrir la conversation")).toBeTruthy();
+
+    fireEvent.change(profileSelect, { target: { value: "qa-assistant" } });
+    fireEvent.click(within(detail).getByText("Ouvrir la conversation"));
+
+    expect(
+      await within(detail).findByLabelText("message à l'assistant"),
+    ).toBeTruthy();
+
+    fireEvent.change(within(detail).getByLabelText("message à l'assistant"), {
+      target: { value: "Crée un plan de correction" },
+    });
+    fireEvent.click(within(detail).getByText("Envoyer"));
+
+    expect(await within(detail).findByText("Crée un plan de correction")).toBeTruthy();
+    expect(
+      await within(detail).findByText(
+        "Assistant: reçu « Crée un plan de correction ».",
+      ),
+    ).toBeTruthy();
+
+    fireEvent.click(within(detail).getByText("Fermer la conversation"));
+
+    await waitFor(() =>
+      expect(
+        within(detail).queryByLabelText("message à l'assistant"),
+      ).toBeNull(),
+    );
+    expect(within(detail).getByText("Ouvrir la conversation")).toBeTruthy();
+    expect(
+      within(detail).queryByText("Crée un plan de correction"),
+    ).toBeNull();
+    expect(
+      within(detail).queryByText("Assistant: reçu « Crée un plan de correction »."),
+    ).toBeNull();
+  });
 });
 
 describe("SprintManager (#11)", () => {