feat(cli): scope Relayfile sandbox mounts (#1630)

* feat(cli): scope Relayfile sandbox mounts

* test(relayflow): prove scoped Relayfile sandbox mounts

* test(relayflow): execute compiled scoped-mount CLI

* style: auto-format with Prettier

* fix(cloud): reject empty Relayfile mount scope

* docs(cli): use sandbox mount root in example

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Co-authored-by: Proactive Runtime Bot <agent@agent-relay.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>