feat(trail): add --repo and --branch overrides for backend/GUI use

entire trail derived the repo from origin and the branch from the current checkout, so it only worked inside a clone of the target repo. Add:

• persistent --repo forge/owner/repo (or a clone URL) on the trail root, honored by show/list/update/delete/watch/finding. Rejected on the local-clone commands (create, checkout, finding apply).
• --branch on show/watch/finding to select a trail by branch instead of the current checkout. update/delete already had --branch. Mutually exclusive with an explicit trail selector.

Auth is unchanged: it already keys off the API base URL's .well-known/entire-api.json, independent of any local git repo.