RepoWorkspace: the GitHub secret becomes a custom bearer entry, so git cannot clone #835

What happens

RepoWorkspace.get_secrets returns
SandboxSecret(name="github", secret_id=..., resource=repo, host=Github.host), where
Github.host = "github.com". SandboxIdentity (sandbox/models.py) turns any ref with a
host into a custom entry: host, auth_variable="GITHUB", auth_header="Authorization",
auth_prefix="Bearer ".

Drukbox's host_secrets.catalog.service() returns a custom Service for every entry with
"host", and so never the catalog github service (with GH_TOKEN, api.github.com,
uploads.github.com, and Basic x-access-token on github.com).

Results:

• docker-sbx: SbxInjection.put_secret calls sbx secret set-custom and not the native
  github secret. git's smart HTTP gets no Basic credential; git clone fails with
  fatal: could not read Username for 'https://github.com'. gh answers 401.
• proxy providers: the box gets GITHUB, not GH_TOKEN, and get_github() does not
  configure git, because GH_TOKEN is not in the env.

The SandboxSecret docstring says "Drukbox knows github.com: that entry is its GitHub
service", which is not what the code does.

Reproduction (docker-sbx)

In a sandbox on the default template, with an installation token minted by
Github.issue_token(repo):

• sbx secret set github --sandbox X --command 'cat <file>' → git clone works, gh works.
• sbx secret set-custom --sandbox X --host github.com --env GITHUB --placeholder P --command 'cat <file>'
  (what Drukbox does for Druks' entry) → git clone asks for a username, gh api user 401.

Possible fixes

1. Druks: RepoWorkspace.get_secrets sends no host for a service that Drukbox's
   catalog knows (github), so the entry is the catalog service.
2. Drukbox: catalog.service() resolves a catalog name before the custom branch, or
   SbxInjection treats a custom entry for github.com as the native secret.

A test that pins the GitHub entry as Drukbox's catalog service would catch a regression.