cacher v0.1.0 — first public release

Replaces the s3_cache_or_curl / s3_cache_docker_image shell helpers in
tarantool-protobuf/.builds/lib/ci-lib.sh with a single static Go binary.

Highlights:
- init/doctor: persist config, smoke-test S3 credentials.
- File caching: download/upload/exists/list/delete, with URL fallback
  and --sha256 verification.
- Docker images: streamed save | zstd → S3 multipart upload, and
  inverse (no on-disk tempfile, pure-Go zstd).
- Directory caching: tar+zstd of resolved trees keyed by lockfile
  hash — closes the gap left by the shell version.
- --hash-from <path> (repeatable) matches sha256sum | cut -c1-N
  byte-for-byte, so existing keys migrate without recomputation.
- Garage-compatible defaults; verified end-to-end against
  dxflrs/garage:v2.3.0 via testcontainers-go.

See CHANGELOG.md for the full surface and the Fixed/Added sections.