Remapping hostname prevents using `gh stack submit`

I use a synthetic hostname mapping in `~/.gitconfig` (1) in conjunction with my `.ssh/config` (2) to select the SSH host key and user identity. In this case, the synthetic hostname is `fastalyze.github.com`. When I run `gh stack submit` this happens:

```
gh stack submit
Checking stack state...
✗ failed to create GitHub client: determining repository: unable to determine current repository, none of the git remotes configured for this repository point to a known GitHub host
```

I suspect there is a check for stacked PR support based on the hostname, and it's not recognizing `fastalyze.github.com`.

(1) `~/.gitconfig` stanza:
```
[url "git@fastalyze.github.com:fastalyze/"]
insteadOf = git@github.com:fastalyze/

[url "git@fastalyze.github.com:fastalyze/"]
insteadOf = https://github.com/fastalyze/
```

(2) `.ssh/config` stanza:
```
host fastalyze.github.com
Hostname github.com
User "jeff-younker"
IdentityFile ~/.ssh/id_fastalyze
IdentityAgent none
```

Start at the `gh stack submit` entry point and reproduce the repository-detection failure using the supplied `.gitconfig` and `.ssh/config` mappings. Trace how the remapped hostname is checked against known GitHub hosts; done when `gh stack submit` recognizes this configuration and proceeds without breaking normal GitHub remote detection.

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