New once cache CLI for reading, writing, and probing the cache

The CLI now exposes once cache blob put, once cache blob get, once cache blob exists, once cache action get, once cache action put, once cache action forget, and once cache stats. Blob operations are content-addressed using BLAKE3 digests. The exists command returns shell-friendly exit codes (0 for hit, 1 for miss) and reports presence in JSON or TOON formats.