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/lib.rs

@@ -12,6 +12,7 @@
 #![forbid(unsafe_code)]
 #![warn(missing_docs)]
 
+pub mod assistant;
 pub mod background_task;
 pub mod clock;
 pub mod conversation;
@@ -39,6 +40,7 @@ pub mod sprints;
 pub mod store;
 pub mod timeparse;
 
+pub use assistant::FsAssistantContextStore;
 pub use background_task::{
     bounded_tail, start_background_ready_inbox_bridge, tail_cap_bytes, BackgroundCompletionSink,
     BackgroundCompletionSinkError, BackgroundCompletionSinkOutcome,
@@ -64,6 +66,7 @@ pub use issues::{FsIssueNumberAllocator, FsIssueStore};
 pub use mailbox::InMemoryMailbox;
 pub use orchestrator::mcp::{
     McpServer, MemoryTransport, StdioTransport, TicketToolError, TicketToolProvider,
+    ToolPolicyRegistry,
 };
 pub use orchestrator::{
     process_request_file, FsOrchestratorWatcher, OrchestratorResponse, OrchestratorWatchHandle,