fix(desktop): correct Grok Build setup guide URL

[armstrongj2001]

Problem

The Grok Build harness preset points its "Setup guide" link at https://build.x.ai/docs, which is unreachable — the hostname has no DNS record at all, so the button leads to a dead page rather than a 404.

$ getent hosts build.x.ai
(no output — no record)

$ getent hosts docs.x.ai
2606:4700::6812:1350  docs.x.ai

$ curl -sS https://build.x.ai/docs
curl: (6) Could not resolve host: build.x.ai

This is user-facing: the link sits behind the Setup guide button in the harness picker, so anyone trying to connect Grok Build hits it while looking for install instructions.

Fix

Point the preset at xAI's live Grok Build documentation.

-        install_instructions_url: "https://build.x.ai/docs",
+        install_instructions_url: "https://docs.x.ai/build/overview",

The preset's command and args are unchanged and remain correct — Grok Build ships native ACP over stdio, which grok agent stdio exposes:

$ grok agent --help
Commands:
  stdio     Run the agent over stdio
  headless  Run the agent headlessly over the Grok WebSocket relay
  ...

Testing

• just ci passes locally (fmt, clippy, desktop lint, Tauri clippy, unit tests, mobile tests).
• Verified https://docs.x.ai/build/overview resolves and returns 200.
• Verified the harness itself still connects: installed Grok Build 0.2.118 via irm https://x.ai/cli/install.ps1 | iex and confirmed grok agent stdio matches the preset's invocation.

No UI changes — this is a string constant, so no screenshots apply.

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