Initial cacher v0.0.1-dev — S3-backed CI cache helper

Replaces tarantool-protobuf/.builds/lib/ci-lib.sh (and its
two shell helpers s3_cache_or_curl + s3_cache_docker_image)
with a single static Go binary.

Commands:
  init / doctor          persist config + smoke-test credentials
  download / upload      single-file cache with URL fallback + sha256
  exists / list / delete cache management
  key                    resolve key template (for shell scripting)
  docker {exists,download,upload}  streamed save/load via zstd
  dir {download,upload}            tar+zstd directory caching (new)

Key derivation via --hash-from <path> (repeatable; file or dir),
matching the existing sha256sum | cut -c1-16 shell convention.
Garage-compatible S3 client (path-style, checksums when_required).