fix(search): derive the default scope from the origin's forge, not a GitHub-only check

entire search required the origin remote to be GitHub-backed before it looked at any flag, so an Entire-native clone (entire://<cluster>/et/...) always failed with "remote is not a GitHub repository" — even with an explicit --repo or --all-repos, and even though the backend serves native repos (the same binary returns et/ results when launched from a GitHub clone).

Mirror the --code path: read the origin's forge-qualified coordinates and seed the default scope with the forge prefix (et/project/repo, gh/owner/repo), so a native repo and a same-named GitHub mirror cannot be conflated by the control plane's "either forge" reading of a bare pair. When an explicit scope is given the origin is not needed at all; without one, an unreadable origin gets the --code hint ("use --repo or --all-repos").

ParseGitHubRemote is unchanged; dispatch keeps its GitHub-only policy.