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/ports.rs

@@ -28,6 +28,7 @@ use async_trait::async_trait;
 use thiserror::Error;
 
 use crate::agent::AgentManifest;
+use crate::agent_tool_policy::AgentToolPolicy;
 use crate::background_task::{
     BackgroundTask, BackgroundTaskKind, BackgroundTaskResult, BackgroundTaskWakePolicy,
 };
@@ -123,6 +124,43 @@ pub struct PreparedContext {
     pub project_root: String,
 }
 
+/// Errors returned while preparing the IdeA-owned ticket assistant context.
+#[derive(Debug, Clone, PartialEq, Eq, Error)]
+pub enum AssistantContextError {
+    /// The context source was missing and no embedded default could be used.
+    #[error("assistant context not found")]
+    NotFound,
+    /// The context could not be read or rendered.
+    #[error("assistant context store failed: {0}")]
+    Store(String),
+}
+
+/// Provides the IdeA-owned context used by ticket assistant sessions.
+#[async_trait]
+pub trait AssistantContextProvider: Send + Sync {
+    /// Renders a prepared context for a ticket assistant bound to `issue`.
+    ///
+    /// # Errors
+    /// [`AssistantContextError`] when the context cannot be read or rendered.
+    async fn prepare_ticket_assistant_context(
+        &self,
+        project: &Project,
+        issue: &Issue,
+    ) -> Result<PreparedContext, AssistantContextError>;
+}
+
+/// Stores requester-scoped MCP tool policies for ephemeral assistant sessions.
+pub trait AgentToolPolicyStore: Send + Sync {
+    /// Sets or replaces the policy for `requester`.
+    fn set_policy(&self, requester: String, policy: AgentToolPolicy);
+
+    /// Returns the policy for `requester`, if any.
+    fn get_policy(&self, requester: &str) -> Option<AgentToolPolicy>;
+
+    /// Clears the policy for `requester`.
+    fn clear_policy(&self, requester: &str);
+}
+
 /// Enriched, **best-effort** details about a conversation, specific to a CLI's
 /// on-disk transcript format (CONTEXT §T6).
 ///