git-remote-entire: actionable hint when the cluster host is missing

git clone entire://gh/entire.io/cli parses gh into the host slot, so the helper previously failed at cluster discovery with an opaque https://gh/.well-known/... error. The empty-host form (entire:///gh/...) hit only a bare "missing host" message.

Detect both shapes in the URL-validation switch: when the host is empty or is a known forge id (gitremote.IsSupportedForge), print a message that names the problem and points at the interactive mirror picker, e.g.

This reuses the existing actionable-error idiom (fatalMessage) and keeps the helper a dumb transport — no TTY/stdin/stdout involvement, and safe in CI (stderr line + exit 128, never a hang). The picker lives in entire repo clone, the right layer, because it clones a fully-qualified URL so later fetches don't re-prompt.