/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.

Inizia dal percorso di output dello slash-command /memory in copilot e segui come l'origin remote viene convertito nella Manage stored memories URL. Riproduci il caso con un origin non GitHub, quindi verifica che il link di gestione venga omesso mentre il caricamento e la gestione delle memories rimangono invariati.

Scritto dal modello di indicizzazione a partire dal testo della issue.