Copilot Chat: Support Enterprise/Organization custom agent modes

[im-aIex]

The web version of Copilot Chat has recently added support for shared custom agent modes at the Enterprise/Organization level by configuring a .github-private repository (note: the documentation around this seems inconsistent and incorrect, I was only able to get it to work with {org}/.github-private repo and an agents root folder).

These custom agents do not currently appear in VSCode when signed in to organization accounts the same as the web version. Ideally, we would be able to share agents, prompts, and instructions using the .github-private repository.

[OrenMe]

I want to support this feature but also add that it is not well formed
Big orgs have many repos in them and they might have sub sets that require certain agents while others require other agents
Putting it all together will create a lot of clutter
We need a way to link agents and prompts and instructions to repos

Also need to be able to pin versions and control distribution so it doesn't necessarily auto pull on each update but also allow notification when an update occurred

[im-aIex]

@OrenMe for per-repo configuration there is also more discussion in #273817.

[graemechristie]

@OrenMe for per-repo configuration there is also more discussion in #273817.

For per repo configuration currently there is just the chat.modeFilesLocations setting which is deprecated, and also requires that the custom agents/chat modes be manually checked out into a location on your local file system.

If the deprecation was removed, and the ability to link directly to a repo in the cloud were added, this would be sufficient. Otherwise this functionality should probably be considered as part of this feature.

Edit: It also seems that the chat.modeFileLocations setting will only find chat modes named using the (now deprecated) *.chatmode.md naming scheme .. it won't recognode custom agents defined in an 'agents' folder.