feat(trail): require explicit branch/selector when --repo is set

With --repo targeting a repo other than the local clone, the branch-defaulting commands (show, update, delete, watch, finding) fell back to the local checkout's current branch, which could silently resolve the wrong trail when a branch name is shared. Require an explicit --branch or trail selector alongside --repo and fail clearly otherwise.

Also restores a //nolint:ireturn directive in agentimport.go that a prior local mise run lint --fix pass had stripped (CI lints without --fix).