feat(tickets): checkpoint backend V1 du système de tickets (T1-T5)

Checkpoint de travail — QA formelle encore à venir (après reset Codex).
`cargo build` workspace OK, tests ticket/issue verts (domaine, store,
use cases, app-tauri 47/51). Le domaine s'appelle `Issue`, exposé
« ticket » côté MCP/UI.

- T1 domaine : entité Issue (statut, priorité, liens, carnet),
  events, ids, ports.
- T2 infra : store FS Markdown + allocator de références #N.
- T3 application : use cases Issue (create/read/list/update/status/
  priority/carnet/link/unlink/assign) + erreurs dédiées.
- T4 surface MCP : 10 outils publics idea_ticket_* (23 outils au
  total), mappés vers les use cases Issue.
- T5 app-tauri : commandes Tauri ticket_* + DTOs d'events Issue +
  câblage state.rs.

Dette de test PRÉ-EXISTANTE héritée de develop (4 tests app-tauri
mcp_e2e_loopback / mcp_serve_peer rouges) hors périmètre tickets,
non traitée ici.

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

crates/infrastructure/tests/mcp_server.rs

@@ -500,6 +500,17 @@ async fn tools_list_advertises_the_idea_tools_with_schemas() {
         // Live-state tools (programme live-state, lot LS4).
         "idea_workstate_read",
         "idea_workstate_set",
+        // Public ticket tools (Issue domain).
+        "idea_ticket_create",
+        "idea_ticket_read",
+        "idea_ticket_list",
+        "idea_ticket_update",
+        "idea_ticket_update_status",
+        "idea_ticket_update_priority",
+        "idea_ticket_read_carnet",
+        "idea_ticket_update_carnet",
+        "idea_ticket_link",
+        "idea_ticket_unlink",
     ] {
         assert!(
             names.contains(&expected),
@@ -509,8 +520,8 @@ async fn tools_list_advertises_the_idea_tools_with_schemas() {
     assert!(!names.contains(&"idea_reply"));
     assert_eq!(
         tools.len(),
-        13,
-        "exactly the thirteen exposed idea_* tools; got {names:?}"
+        23,
+        "exactly the twenty-three exposed idea_* tools; got {names:?}"
     );
 
     // Every tool advertises an object input schema.