dispatch: address a checkout by its origin's forge, not GitHub by assumption

entire dispatch in an Entire-native checkout failed before any request left the client: "dispatch currently supports GitHub origin remotes only: remote is not a GitHub repository (host: aws-us-east-2.entire.io)". The same dispatch succeeds with --repos et/<project>/<repo>, so the server has supported native repos all along; the origin-derived default parsed origin with a GitHub-only parser and then stamped gh/ unconditionally.

The forge is already in the remote URL and gitremote.ParseURL already reads it (github.com and entire://…/gh/… are gh; entire://…/et/… is et). OriginRepoSlug turns that into the forge-qualified slug --repos accepts, and the three places that derived a repo from origin use it:

• cloud mode's implicit default sends the slug as-is;
• local mode names the repo group by it (GitHub repos keep their bare name and github.com link; native repos show as et/… with no link);
• the wizard's on-disk discovery offers native checkouts instead of dropping them.

An origin on a host Entire does not serve is still refused, and the error now names the --repos shapes. search.ParseGitHubRemote has no remaining caller and goes with its tests.