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/infrastructure/src/orchestrator/mcp/mod.rs

@@ -29,6 +29,7 @@
 //! child process).
 
 pub mod jsonrpc;
+pub mod policy;
 pub mod server;
 pub mod tickets;
 pub mod tools;
@@ -37,6 +38,7 @@ pub mod transport;
 pub use jsonrpc::{
     JsonRpcError, JsonRpcRequest, JsonRpcResponse, Transport, TransportError, JSONRPC_VERSION,
 };
+pub use policy::ToolPolicyRegistry;
 pub use server::McpServer;
 pub use tickets::{TicketToolError, TicketToolProvider};
 pub use tools::{catalogue, map_tool_call, tool_returns_reply, ToolDef, ToolMapError};