Add end-to-end test suite against real Garage via testcontainers

* internal/testutil/garage: spins up dxflrs/garage:v2.3.0 with
  --single-node --default-bucket so the bucket + access key are
  auto-created from env vars at startup, no CLI bootstrap dance.
  Each Start() call yields a fresh container with random creds and
  registers t.Cleanup teardown.

* e2e_test.go gated by build tag e2e exercises the compiled cacher
  binary against the container — covers init/doctor parity (the
  regression guard for the HeadBucket+signature bugs we hit on
  the real bucket), single-file round-trip, exit codes (1/2/3),
  URL fallback + cache fill, --hash-from parity with sha256sum,
  directory tar+zstd round-trip, and delimited list output.

* just test-e2e recipe; requires Docker on the host.

Total runtime ~30s after first image pull (~1.5s/container).

Fix Garage compat: PingBucket via ListObjects, fixed config path, delimited list

Three issues surfaced when first running doctor against the real
s3.bigb.es Garage instance:

* HeadBucket returned 403 even with valid creds. Garage doesn't
  implement HeadBucket the way AWS does. Switch the doctor smoke
  test to a 1-key ListObjectsV2 — matches what aws s3 ls does, which
  the existing shell helper used.

* Config landed in ~/Library/Application Support/cacher on macOS
  via os.UserConfigDir. cacher is a CI tool that needs the same
  path on a dev mac and on a Linux build runner, so hard-code
  ~/.config/cacher/config.toml unconditionally.

* cacher list flat-listed every object under the configured prefix.
  Add Delimiter="/" semantics by default (matches aws s3 ls), with
  --recursive for the previous behaviour and --root to ignore the
  configured prefix and list at the bucket root.

Add BSD-2-Clause license and README

Copyright header matches the convention used in go-luarocks.
README walks through install, init/doctor, single-file and docker
caching, the new directory caching, key derivation, config
precedence, Garage compatibility, and exit codes.

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).