feat: fork codex threads

tests/fixtures/codex-app-server-runtime.mjs

@@ -705,6 +705,32 @@ rl.on("line", (line) => {
     return;
   }
 
+  if (message.method === "thread/fork") {
+    send({
+      id: message.id,
+      result: {
+        thread: {
+          id: `${message.params?.threadId}-fork`,
+          sessionId: "fork-session",
+          forkedFromId: message.params?.threadId,
+          name: "Forked working thread",
+          preview: "Fork preview should be safe",
+          ephemeral: message.params?.ephemeral === true,
+          status: "idle",
+          path: "/private/path/that-should-not-leak",
+          cwd: "/private/cwd/that-should-not-leak",
+          turns: [],
+          internalForkSecret: "thread-fork-secret-should-not-leak",
+        },
+        model: "gpt-5.4",
+        modelProvider: "openai",
+        cwd: "/private/cwd/that-should-not-leak",
+        instructionSources: ["/private/AGENTS.md"],
+      },
+    });
+    return;
+  }
+
   if (message.method === "thread/read") {
     send({
       id: message.id,