fix(tickets): normalise assignedAgentIds/links/attachments manquants sur les payloads legacy

Les lignes de ticket persistées avant l'introduction de ces champs peuvent
arriver sans assignedAgentIds (ni links/attachments), ce que TicketsPanel
déréférence sans garde côté rendu : la fenêtre de liste rend quelques lignes
puis devient noire dès qu'une ligne legacy est itérée. Les deux gateways
(Tauri et HTTP/stream) normalisent désormais chaque ticket et chaque item de
liste avant de les retourner à l'UI.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/adapters/ticket.test.ts

@@ -55,6 +55,26 @@ describe("TauriTicketGateway invoke payloads", () => {
     });
   });
 
+  it("normalizes legacy ticket list rows that omit assignedAgentIds", async () => {
+    invoke.mockResolvedValueOnce({
+      items: [
+        {
+          ref: "#12",
+          path: ".ideai/tickets/12/issue.md",
+          title: "Legacy row",
+          status: "open",
+          priority: "medium",
+          createdBy: { kind: "user" },
+          updatedAt: 1,
+        },
+      ],
+    });
+
+    const list = await new TauriTicketGateway().list("proj-1");
+
+    expect(list.items[0]?.assignedAgentIds).toEqual([]);
+  });
+
   it("wraps ticket attachment commands in the request DTO", async () => {
     const gateway = new TauriTicketGateway();
 
@@ -97,4 +117,27 @@ describe("TauriTicketGateway invoke payloads", () => {
       },
     );
   });
+
+  it("normalizes legacy ticket payloads that omit array fields", async () => {
+    invoke.mockResolvedValueOnce({
+      id: "issue-1",
+      ref: "#12",
+      number: 12,
+      title: "Legacy",
+      description: "",
+      status: "open",
+      priority: "medium",
+      createdBy: { kind: "user" },
+      updatedBy: { kind: "user" },
+      createdAt: 1,
+      updatedAt: 1,
+      version: 1,
+    });
+
+    const ticket = await new TauriTicketGateway().read("proj-1", "#12", true);
+
+    expect(ticket.links).toEqual([]);
+    expect(ticket.assignedAgentIds).toEqual([]);
+    expect(ticket.attachments).toEqual([]);
+  });
 });