feat(codex): add the codexConfig API namespace

Adds a typed client wrapper for the Codex config API alongside ccConfig,
with methods for reading/writing skills, agents, rules, memory, config,
MCP servers, plugins, and marketplaces. Mirrors ccConfig construction and
error handling. All response shapes match the server's wire format.

client/src/lib/__tests__/codexConfigApi.test.ts

@@ -0,0 +1,47 @@
+/**
+ * @file Pins the codexConfig API routes and payloads. Tests that the client
+ * sends the right method, URL, and body shape to the Codex config backend.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { api } from "../api";
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+function stubFetch(response: unknown) {
+  const spy = vi.fn().mockResolvedValue({
+    ok: true,
+    status: 200,
+    headers: { get: () => "application/json" },
+    json: async () => response,
+    text: async () => JSON.stringify(response),
+  });
+  vi.stubGlobal("fetch", spy);
+  return spy;
+}
+
+describe("api.codexConfig", () => {
+  it("sends a write as a single PUT with the artifact envelope", async () => {
+    const spy = stubFetch({ ok: true, file: "/x", created: true });
+
+    await api.codexConfig.write({
+      scope: "user",
+      type: "rules",
+      name: "default",
+      content: "allow",
+    });
+
+    expect(spy).toHaveBeenCalledTimes(1);
+    const [, options] = spy.mock.calls[0] as [string, RequestInit | undefined];
+    expect(options?.method).toBe("PUT");
+    expect(JSON.parse(options?.body as string)).toEqual({
+      scope: "user",
+      type: "rules",
+      name: "default",
+      content: "allow",
+    });
+  });
+});