Guide users to enable coding agent from GHPRI

[bamurtaugh]

I opened an issue in VS Code via GHPRI and wanted to assign Copilot to it:

I was confused why I didn't see Copilot as an option, but I realize I didn't have Coding Agent enabled for this repo.

It'd be great to guide users to enable the Coding Agent from the Assignees quick pick, and perhaps other entry points in GHPRI (i.e. PR or issues viewlets themselves).

[alexr00]

Let's see if we have GH API that will let us do this...

[alexr00]

Closing, as we have #7302 to track this and users who can actually do something about turning coding agent on would have had to turn it off manually to end up in this state.