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/adapters/mock/index.ts

@@ -35,12 +35,14 @@ import type {
   ProfileAvailability,
   ResumableAgent,
   Skill,
+  ReplyChunk,
   SkillScope,
   Sprint,
   Template,
   TerminalSession,
   Ticket,
   TicketCarnet,
+  TicketChat,
   TicketLink,
   TicketLinkKind,
   TicketList,
@@ -1816,6 +1818,9 @@ export class MockTicketGateway implements TicketGateway {
   private counters = new Map<string, number>();
   private sprints = new Map<string, Map<string, Sprint>>();
   private sprintCounters = new Map<string, number>();
+  /** Open assistant chats, keyed by `issueRef` → sessionId (ticket #8). */
+  private chatSessions = new Map<string, string>();
+  private chatCounter = 0;
 
   constructor(private readonly system?: MockSystemGateway) {}
 
@@ -2235,6 +2240,51 @@ export class MockTicketGateway implements TicketGateway {
     this.system?.emit({ type: "sprintDeleted", sprintId });
   }
 
+  async openTicketChat(
+    projectId: string,
+    issueRef: string,
+    profileId: string,
+  ): Promise<TicketChat> {
+    // Ensure the ticket exists (mirrors the backend resolving the ref).
+    this.require(projectId, issueRef);
+    const sessionId = `mock-ticket-chat-${++this.chatCounter}`;
+    this.chatSessions.set(issueRef, sessionId);
+    this.system?.emit({ type: "ticketAssistantOpened", issueRef, profileId });
+    return { sessionId, requester: "ticket-assistant", issueRef };
+  }
+
+  async closeTicketChat(_projectId: string, issueRef: string): Promise<void> {
+    if (this.chatSessions.delete(issueRef)) {
+      this.system?.emit({ type: "ticketAssistantClosed", issueRef });
+    }
+  }
+
+  async sendTicketChat(
+    sessionId: string,
+    message: string,
+    onChunk: (chunk: ReplyChunk) => void,
+  ): Promise<void> {
+    if (![...this.chatSessions.values()].includes(sessionId)) {
+      throw {
+        code: "NOT_FOUND",
+        message: `chat session ${sessionId} not found`,
+      } as GatewayError;
+    }
+    // Simulate a streamed assistant turn: a couple of text deltas, then the
+    // deterministic `final` chunk. Delivered across microtasks (like a real
+    // Channel), so a consumer that awaits sees the whole turn.
+    const content = `Assistant: reçu « ${message} ».`;
+    const chunks: ReplyChunk[] = [
+      { kind: "textDelta", text: "Assistant: reçu " },
+      { kind: "textDelta", text: `« ${message} ».` },
+      { kind: "final", content },
+    ];
+    for (const chunk of chunks) {
+      await Promise.resolve();
+      onChunk(chunk);
+    }
+  }
+
   private requireSprint(projectId: string, sprintId: string): Sprint {
     const sprint = this.projectSprints(projectId).get(sprintId);
     if (!sprint) {