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/ports/index.ts

@@ -37,12 +37,14 @@ import type {
   ProjectWorkState,
   ProfileAvailability,
   ResumableAgent,
+  ReplyChunk,
   Skill,
   SkillScope,
   Sprint,
   Template,
   TerminalSession,
   Ticket,
+  TicketChat,
   TicketCarnet,
   TicketLinkKind,
   TicketList,
@@ -829,6 +831,29 @@ export interface TicketGateway {
    * them); they fall back to the "no sprint" bucket (#11).
    */
   deleteSprint(projectId: string, sprintId: string): Promise<void>;
+  /**
+   * Opens an AI assistant chat bound to a ticket (ticket #8). Spawns a
+   * structured assistant session driven by `profileId`; returns the live
+   * session handle. One session per ticket — reopening supersedes.
+   */
+  openTicketChat(
+    projectId: string,
+    issueRef: string,
+    profileId: string,
+  ): Promise<TicketChat>;
+  /** Closes (disposes) a ticket's assistant chat session (ticket #8). */
+  closeTicketChat(projectId: string, issueRef: string): Promise<void>;
+  /**
+   * Sends a message on an open assistant session and streams the reply back as
+   * {@link ReplyChunk}s via `onChunk` (ticket #8). Resolves once the turn has
+   * started; the turn ends at the `final` chunk. Reuses the shared `agent_send`
+   * chat transport (the same as the structured chat cell).
+   */
+  sendTicketChat(
+    sessionId: string,
+    message: string,
+    onChunk: (chunk: ReplyChunk) => void,
+  ): Promise<void>;
 }
 
 /**