Chat customization: promote the built-in Github MCP server

[aeschli]

Testing #303656

• set github.copilot.chat.githubMcpServer.readonly to true. This add the built-in GitHub MCP server
• open chat customizations
• the GitHub MCP server is not shown

-> should show and offer easy ways to select the variant (readonly...)
-> if not yet added should offer an easy way to add it. I would say it should be always there, with a checkbox to enable/disable

[joshspicer]

I did not realize that if enabled is false, any of the other settings have effect. That seems to go against our usual convention. Is that definitely the case?

Good feedback on the toggling. Will look into always showing it as a 'disabled' form when the setting(s) are off to make enabling straightforward

[aeschli]

oh, I think you are right. I should have checked the first check box as well. I just wanted to enable the read-only variant.
FYI Tyler James Leonhardt (@TylerLeonhardt) maybe this could just be a drop-down with the different variants.

[TylerLeonhardt]

Well, the intent is that the enabled setting goes away and it's just always available... but we can't do that until it doesn't hit COGS.