Add `entire trail checkout` subcommand

Check out the branch of a trail, resolved from the first argument or --trail (number, id, or branch), or the current branch when omitted.

• Looks the trail up against the current clone's origin remote, so a resolved trail inherently belongs to the repo you run it from.
• Reuses switchToBranchForResume for the local-vs-remote checkout, the uncommitted-changes guard, and the fetch prompt (--force skips it).
• Extracts a shared resolveTrailBySelector helper and refactors runTrailShow to use it.