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/domain/index.ts

@@ -187,6 +187,21 @@ export type DomainEvent =
       version: number;
     }
   | { type: "sprintDeleted"; sprintId: string }
+  | {
+      /**
+       * A ticket AI assistant chat was opened (ticket #8). Mirror of the backend
+       * `DomainEventDto::TicketAssistantOpened`. `issueRef` is the bound ticket,
+       * `profileId` the runtime AI profile driving the assistant.
+       */
+      type: "ticketAssistantOpened";
+      issueRef: string;
+      profileId: string;
+    }
+  | {
+      /** A ticket AI assistant chat was closed/disposed (ticket #8). */
+      type: "ticketAssistantClosed";
+      issueRef: string;
+    }
   | {
       /**
        * An intermediate assistant announcement emitted **during** an inter-agent
@@ -963,3 +978,25 @@ export interface TicketCarnet {
   carnet: string;
   version: number;
 }
+
+/**
+ * A ticket AI assistant chat session (mirror of the backend `TicketChatDto`,
+ * ticket #8). `sessionId` is the live structured session driving the assistant;
+ * `requester` is the assistant agent id; `issueRef` the bound ticket.
+ */
+export interface TicketChat {
+  sessionId: string;
+  requester: string;
+  issueRef: TicketRef;
+}
+
+/**
+ * One streamed chunk of an assistant turn (mirror of the backend `ReplyChunk`,
+ * tagged on `kind`). `textDelta` is an incremental text fragment; `toolActivity`
+ * a best-effort activity badge; `final` the deterministic end-of-turn chunk
+ * carrying the aggregated content — after it the turn is frozen.
+ */
+export type ReplyChunk =
+  | { kind: "textDelta"; text: string }
+  | { kind: "toolActivity"; label: string }
+  | { kind: "final"; content: string };