New runtime cache CLI lets scripts probe and reuse the cache directly

Once now ships a lower-level once cache CLI that any script can call to probe, read, or write the cache directly. Commands include blob put/get/exists, action get/put/forget, and stats. This lets shell and other scripts short-circuit work when inputs have not changed, without needing to be rewritten as build rules. The CLI is documented in the new Cache CLI guide, including examples for skipping a test run, restoring a tarball such as node_modules, and caching a mise-managed toolchain.