Copilot app: no way to sign in again to an MCP server whose sign-in expired (/mcp auth hint doesn't work, Customize tab shows green) #5175

What happens

1. The FabricIQ MCP server (from the fabric-skills plugin) needs me to sign in again.
2. The agent says: "To sign in again, run /mcp auth FabricIQ, then tell me to continue."
3. The app has no /mcp command, so I can't follow that instruction.
4. Settings → Customize → MCP Servers shows FabricIQ with a green check, and there's no option to sign in again.
5. The only fix is to open the Copilot CLI and run /mcp auth FabricIQ there.

What I expected

• The app's instruction should be something I can actually do in the app, such as a "Sign in" button or link.
• The Customize tab should show that the server needs sign-in (not green) and offer a "Sign in again" button.

Environment

• GitHub Copilot app 1.0.99-2, Windows
• MCP server: FabricIQ (Plugin: fabric-skills). The same likely applies to other sign-in-based MCP servers such as workiq.

Related CLI issues: #2779 (renew sign-ins automatically), #4818