Compressed cache storage and a richer runtime cache CLI

Cache blobs are now stored compressed with zstd, which reduces disk usage while keeping the same BLAKE3-based addressing. The cache CLI also exposes a new once cache blob exists probe and a cache hash helper for files and directories. When you declare action cache entries, you can use repeatable --input flags that combine file paths, directory trees, literal values, environment variables, and stdin. Content is still addressed by digest, and the compressed store stays compatible with previously stored raw blobs.