refactor how remote repo/file providers represent context

[sqs]

Now, they just embed a ContextItem in their mention.data.contextItem field. This eliminates ad-hoc intermediate ways of representing a remote repo or file, of which there were a few in the codebase.

No behavior change.

Test plan

Perform remote @-repo and @-file mentions in VS Code and Cody Web. Ensure they work. In particular, ensure the right context is fetched and that remote @-file mentions have the 2-step input (where first you pick the repo and then the file).