Add Grok Build to the ACP provider catalog

Verified the ACP entrypoint against a real install (grok 0.2.11): the
command is `grok agent stdio`, and an initialize handshake returns a
valid ACP response (protocolVersion 1, model grok-build). Running
prompts requires a SuperGrok or X Premium+ login.

packages/app/src/data/acp-provider-catalog.ts

@@ -231,6 +231,16 @@ const CATALOG_DATA = [
     installLink: "https://block.github.io/goose/",
     command: ["goose", "acp"],
   },
+  {
+    id: "grok",
+    title: "Grok",
+    description:
+      "xAI's Grok Build agentic coding CLI with plan mode and parallel subagents. Requires a SuperGrok or X Premium+ subscription.",
+    version: "0.2.11",
+    iconId: "grok",
+    installLink: "https://docs.x.ai/build/overview",
+    command: ["grok", "agent", "stdio"],
+  },
   {
     id: "hermes",
     title: "Hermes",