fix(trail): reject host-like --repo forge in the bare path form

The bare forge/owner/repo form accepted parts[0] as the forge without checking it is a known short forge id, so --repo github.com/acme/app (or gitlab.com/...) slipped through and produced a malformed /api/v1/trails/<forge>/... path that failed with an opaque server error.

Validate the forge segment against gitremote.IsSupportedForge and fail with a clear message pointing at a forge id ("gh") or a clone URL.