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/src/orchestrator/mcp/tools.rs

@@ -61,7 +61,13 @@ pub fn tool_returns_reply(tool: &str) -> bool {
             | "idea_memory_read"
             | "idea_skill_read"
             | "idea_workstate_read"
-    )
+    ) || is_ticket_tool(tool)
+}
+
+/// Whether `tool` is a public ticket MCP tool.
+#[must_use]
+pub fn is_ticket_tool(tool: &str) -> bool {
+    super::tickets::is_ticket_tool(tool)
 }
 
 /// The full catalogue advertised on `tools/list`.
@@ -70,7 +76,7 @@ pub fn tool_returns_reply(tool: &str) -> bool {
 /// [`OrchestratorCommand`].
 #[must_use]
 pub fn catalogue() -> Vec<ToolDef> {
-    vec![
+    let mut tools = vec![
         ToolDef {
             name: "idea_list_agents",
             description: "List the IdeA agents declared in the project's manifest. Returns the \
@@ -255,7 +261,9 @@ pub fn catalogue() -> Vec<ToolDef> {
                 "additionalProperties": false
             }),
         },
-    ]
+    ];
+    tools.extend(super::tickets::catalogue());
+    tools
 }
 
 /// Maps an MCP `tools/call` (`name` + `arguments`) to a validated