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/events.rs

@@ -183,6 +183,18 @@ pub enum DomainEvent {
         /// Exit code.
         code: i32,
     },
+    /// A ticket assistant session was opened for an issue.
+    TicketAssistantOpened {
+        /// Bound issue.
+        issue_ref: IssueRef,
+        /// Runtime profile used by the assistant.
+        profile_id: ProfileId,
+    },
+    /// A ticket assistant session was closed for an issue.
+    TicketAssistantClosed {
+        /// Bound issue.
+        issue_ref: IssueRef,
+    },
     /// An agent's **busy/idle** state changed (cadrage C4 §4.2): it went `Busy` on
     /// the enqueue that started a turn, or `Idle` on `mark_idle` (prompt-ready or an
     /// explicit signal). Discrete, low-frequency beacon relayed to the front so the
@@ -531,6 +543,14 @@ mod tests {
         AgentId::from_uuid(uuid::Uuid::from_u128(n))
     }
 
+    fn profile(n: u128) -> ProfileId {
+        ProfileId::from_uuid(uuid::Uuid::from_u128(n))
+    }
+
+    fn issue_ref(n: u64) -> IssueRef {
+        IssueRef::from(crate::IssueNumber::new(n).unwrap())
+    }
+
     // -- §21 : constructibilité + égalité PartialEq des 5 variantes --------------
 
     #[test]
@@ -638,6 +658,47 @@ mod tests {
         );
     }
 
+    #[test]
+    fn ticket_assistant_opened_constructs_and_compares() {
+        let ev = DomainEvent::TicketAssistantOpened {
+            issue_ref: issue_ref(7),
+            profile_id: profile(9),
+        };
+        assert_eq!(
+            ev,
+            DomainEvent::TicketAssistantOpened {
+                issue_ref: issue_ref(7),
+                profile_id: profile(9),
+            }
+        );
+        assert_ne!(
+            ev,
+            DomainEvent::TicketAssistantOpened {
+                issue_ref: issue_ref(8),
+                profile_id: profile(9),
+            }
+        );
+    }
+
+    #[test]
+    fn ticket_assistant_closed_constructs_and_compares() {
+        let ev = DomainEvent::TicketAssistantClosed {
+            issue_ref: issue_ref(7),
+        };
+        assert_eq!(
+            ev,
+            DomainEvent::TicketAssistantClosed {
+                issue_ref: issue_ref(7),
+            }
+        );
+        assert_ne!(
+            ev,
+            DomainEvent::TicketAssistantClosed {
+                issue_ref: issue_ref(8),
+            }
+        );
+    }
+
     #[test]
     fn distinct_session_limit_variants_are_not_equal() {
         // Les variantes ne se confondent pas entre elles malgré des champs proches.