Open GitHub Desktop from CLI

### Describe the feature or problem you'd like to solve

Add a slash command to open GitHub Desktop in the current working directory

### Proposed solution

Just like `/ide` opens an IDE workspace, `/github` (or some variation, like `/desktop`) opens GitHub Desktop in the current working directory. This is mostly just an alternative to `! github .` with less silly output in the log (as each time it prints `└ No Content`).

I looked for issues mentioning GitHub Desktop, but none seemed related to this suggestion.

More generally, users could configure their Git GUI of choice.

### Example prompts or workflows

`/github`
`/desktop`
`/git`

### Additional context

_No response_

Start by locating the existing /ide slash-command implementation and its current-working-directory handling. Add a comparable /github or /desktop command that opens GitHub Desktop in that directory, then verify the command works without the repeated `No Content` log output.

索引モデルが issue の本文から書いたものです。