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>

crates/application/src/lib.rs

@@ -30,6 +30,7 @@ pub mod skill;
 pub mod sprints;
 pub mod template;
 pub mod terminal;
+pub mod ticket_assistant;
 pub mod window;
 pub mod workstate;
 
@@ -149,6 +150,10 @@ pub use terminal::{
     ResizeTerminal, ResizeTerminalInput, StructuredSessions, TerminalSessions, WriteToTerminal,
     WriteToTerminalInput,
 };
+pub use ticket_assistant::{
+    CloseTicketAssistant, CloseTicketAssistantInput, OpenTicketAssistant, OpenTicketAssistantInput,
+    OpenTicketAssistantOutput,
+};
 pub use window::{MoveTabToNewWindow, MoveTabToNewWindowInput, MoveTabToNewWindowOutput};
 pub use workstate::{
     AgentBackgroundTaskState, AgentTicketState, AgentWorkState, AttachLiveAgent,