Feature Request: /remote should support non-GitHub git repositories (e.g. GitLab, Bitbucket)

## Problem
`/remote` fails with `Remote session disabled: not in a GitHub repository` when used in a GitLab (or other) repository.

## Proposed Solution
The remote session feature should work independently of the git host — session control via GitHub account, regardless of where the code is hosted.

## Use Case
Teams using GitLab want to use /remote to control their Copilot CLI session from GitHub web/mobile, but are blocked by the repository host check.

## Additional Context
The CLI is already authenticated via GitHub, so the restriction to GitHub-hosted repos feels unnecessary for the remote control functionality itself.

Search the CLI for the exact error message and the /remote command, then trace the repository-host check and its tests. Confirm how GitHub authentication controls remote sessions, and define done as allowing GitLab or other Git repositories without breaking the existing GitHub-hosted flow.

Written by the indexing model from the issue text.