Cursor/Grok Bot marketplace plugin: Authenticate fails with "Github didn't provide a sign-in link"

## Summary
In Cursor / Grok Bot, the official GitHub marketplace MCP plugin fails OAuth: account stays **Needs auth**, and clicking **Authenticate** shows toast **"Github didn't provide a sign-in link"** (no browser / no authorize URL).

## Environment
- Grok Bot desktop **0.44.0** stable, darwin arm64
- Cursor IDE **3.19.13** arm64 (`dd066f332fcea7382764400fde902f61920648d0`)
- Marketplace plugin id **48677658** (remote MCP `user-Github`: needsAuth / authentication_required)

## Steps
1. Install/open GitHub plugin in Grok Bot desktop Plugins.
2. Account `default` shows Needs auth.
3. Click Authenticate.
4. Toast: Github didn't provide a sign-in link; still Needs auth.

## Expected
Authenticate should return an OAuth authorization URL / open the GitHub sign-in flow.

## Notes
- Related Cursor forum (screenshot): https://forum.cursor.com/t/grok-bot-github-plugin-authenticate-github-didnt-provide-a-sign-in-link/170898
- Related-but-distinct Cursor IDE OAuth UX thread: https://forum.cursor.com/t/plugin-mcp-oauth-never-reaches-needsauth-authenticate-button-missing-browser-never-opens/170058
- Local workaround: PAT-backed custom MCP (`github-local`); marketplace OAuth path still broken.

Happy to provide more client logs if useful.

Reproduce the OAuth failure in the GitHub marketplace MCP plugin using the listed Grok Bot and Cursor versions, then inspect the available client or server logs for the missing sign-in URL. Compare the marketplace OAuth path with the documented PAT-backed custom MCP workaround; done means Authenticate returns an authorization URL and completes the GitHub sign-in flow.

索引モデルが issue の本文から書いたものです。