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/ticket.ts

@@ -10,13 +10,15 @@
  * {@link isTicketVersionConflict} to branch on it in the UI.
  */
 
-import { invoke } from "@tauri-apps/api/core";
+import { Channel, invoke } from "@tauri-apps/api/core";
 
 import type {
   GatewayError,
+  ReplyChunk,
   Sprint,
   Ticket,
   TicketCarnet,
+  TicketChat,
   TicketLinkKind,
   TicketList,
 } from "@/domain";
@@ -182,4 +184,38 @@ export class TauriTicketGateway implements TicketGateway {
       request: { projectId, sprintId },
     });
   }
+
+  async openTicketChat(
+    projectId: string,
+    issueRef: string,
+    profileId: string,
+  ): Promise<TicketChat> {
+    return invoke<TicketChat>("open_ticket_chat", {
+      request: { projectId, issueRef, profileId },
+    });
+  }
+
+  async closeTicketChat(projectId: string, issueRef: string): Promise<void> {
+    await invoke<void>("close_ticket_chat", {
+      request: { projectId, issueRef },
+    });
+  }
+
+  async sendTicketChat(
+    sessionId: string,
+    message: string,
+    onChunk: (chunk: ReplyChunk) => void,
+  ): Promise<void> {
+    // Per-turn reply channel, tagged on `kind` (`textDelta`|`toolActivity`|
+    // `final`). Reuses the shared `agent_send` chat transport (the same command
+    // and ChatBridge the structured chat cell uses). Wire the handler before the
+    // invoke so no early chunk is missed.
+    const channel = new Channel<ReplyChunk>();
+    channel.onmessage = (chunk) => onChunk(chunk);
+    await invoke<void>("agent_send", {
+      sessionId,
+      prompt: message,
+      onReply: channel,
+    });
+  }
 }