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/domain/src/lib.rs

@@ -31,6 +31,7 @@
 #![warn(missing_docs)]
 
 pub mod agent;
+pub mod agent_tool_policy;
 pub mod background_task;
 pub mod conversation;
 pub mod conversation_log;
@@ -79,6 +80,8 @@ pub use project::{Project, ProjectPath};
 
 pub use agent::{Agent, AgentManifest, AgentOrigin, ManifestEntry};
 
+pub use agent_tool_policy::AgentToolPolicy;
+
 pub use background_task::{
     BackgroundTask, BackgroundTaskError, BackgroundTaskKind, BackgroundTaskResult,
     BackgroundTaskState, BackgroundTaskWakePolicy, BACKGROUND_TASK_LABEL_MAX_CHARS,
@@ -180,7 +183,8 @@ pub use orchestrator::{
 };
 
 pub use ports::{
-    AgentContextStore, AgentRuntime, BackgroundCompletionStream, BackgroundTaskCompletion,
+    AgentContextStore, AgentRuntime, AgentToolPolicyStore, AssistantContextError,
+    AssistantContextProvider, BackgroundCompletionStream, BackgroundTaskCompletion,
     BackgroundTaskHandle, BackgroundTaskPortError, BackgroundTaskRunner, BackgroundTaskSpec,
     BackgroundTaskStore, Clock, ContextInjectionPlan, DirEntry, Embedder, EmbedderEnvInspector,
     EmbedderEnvReport, EmbedderError, EmbedderProfileStore, EmbedderPromptDismissal,