/memory shows invalid "Manage stored memories" link for non-GitHub repositories (404s)

### Describe the bug

When I run the `/memory` slash command inside a repository whose `origin` remote is **not** GitHub (in my case, an Azure DevOps repo), the command's output includes a `Manage stored memories: https://github.com///...` link.

That URL is constructed from the local git origin's path and points at github.com, but the repo doesn't exist on GitHub — so the link opens a 404 page.

The link shouldn't be shown at all when the current repository's remote isn't hosted on GitHub.

### Affected version

1.0.49-6

### Steps to reproduce the behavior

1. `cd` into any local git repository whose `origin` remote points at a non-GitHub host (e.g., `https://.visualstudio.com//_git/` for Azure DevOps, or any GitLab/Bitbucket clone).
2. Launch `copilot`.
3. Run `/memory` at the interactive prompt.
4. Observe the output.

### Expected behavior

The `Manage stored memories: …` link is omitted entirely when the current repo's origin isn't a `github.com` URL, since the URL the CLI would generate isn't valid in any other host's UI.

Ideally the CLI would detect the host of the `origin` remote and only emit a management link when it knows where the memory store actually lives on that host (today that appears to be GitHub-only).

### Additional context

- OS: Windows 11 (PowerShell 7)
- The CLI itself works fine — memories are still being managed/loaded as expected. This is purely a misleading output artifact from `/memory`.
- Repro is independent of whether memory is currently enabled or disabled for the session.