feat(tickets): suppression d'un ticket (backend + frontend)

Ajoute la suppression complète d'un ticket (#6).

Backend Rust :
- port IssueStore::delete (NotFound si absent) + event IssueDeleted{freed_sprint}
- FsIssueStore::delete : supprime .ideai/tickets/<N>/ et l'index sous lock
- use case DeleteIssue + adaptations des tests sprint/ticket_assistant au port
- commande Tauri ticket_delete + câblage events/state/lib

Frontend :
- gateway delete (ports, adapter ticket + mock, domain)
- useTicketDetail : retrait de la liste et fermeture du détail via event issueDeleted
- intégration TicketDetail + tests

Tests verts : application/issue_usecases (6), infrastructure/issue_store (7),
app-tauri --lib (56), frontend vitest (503), npm run build (exit 0).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/infrastructure/tests/issue_store.rs

@@ -182,6 +182,46 @@ async fn issue_store_persists_and_filters_sprint_membership() {
     assert_eq!(rows[0].sprint, Some(sprint_id));
 }
 
+#[tokio::test]
+async fn issue_store_delete_removes_files_and_index_entry() {
+    let tmp = TempDir::new();
+    let root = tmp.root();
+    let store = FsIssueStore::new();
+    store
+        .create(&root, &issue(&root, 1, "Delete me"))
+        .await
+        .unwrap();
+    std::fs::write(tmp.child(".ideai/tickets/1/attachment.txt"), "payload").unwrap();
+
+    store
+        .delete(&root, IssueRef::from_str("#1").unwrap())
+        .await
+        .unwrap();
+
+    assert!(!tmp.child(".ideai/tickets/1/issue.md").exists());
+    assert!(!tmp.child(".ideai/tickets/1/carnet.md").exists());
+    assert!(!tmp.child(".ideai/tickets/1/attachment.txt").exists());
+    assert!(!tmp.child(".ideai/tickets/1").exists());
+    let rows = store.list(&root, IssueListFilter::default()).await.unwrap();
+    assert!(rows.is_empty());
+    let index = std::fs::read_to_string(tmp.child(".ideai/tickets/index.json")).unwrap();
+    assert!(!index.contains("\"issueRef\": \"#1\""));
+}
+
+#[tokio::test]
+async fn issue_store_delete_returns_not_found_when_absent() {
+    let tmp = TempDir::new();
+    let root = tmp.root();
+    let store = FsIssueStore::new();
+
+    let err = store
+        .delete(&root, IssueRef::from_str("#404").unwrap())
+        .await
+        .unwrap_err();
+
+    assert!(matches!(err, IssueStoreError::NotFound));
+}
+
 #[tokio::test]
 async fn allocator_never_reuses_numbers() {
     let tmp = TempDir::new();