Replace trail runner with rerun-review and agent readings

Trail runners were replaced by project agents (#2946), so trail runner is gone. In its place:

• trail rerun-review [--agent <id>] reviews the change's current head again. Without --agent it uses the change's review-rerun, which starts the project's agents and the repository's own reviewers; with --agent it starts one project agent through the project's agent-run route, naming the trail and change.
• trail finding shows each project agent's latest reading and run, or what it waits for, from the change detail's agents; --json passes the server's items through.

update-branch, ci-pr and rerun-review now refuse to guess on a trail with several open changes, and update-branch and ci-pr take --json. Merge and these commands share one repository-ID path helper.