merge .builds/{unit,e2e}.yml into test.yml; cache go + garage image

Single job runs unit tests → smoke build → e2e (against a real Garage
container via testcontainers-go). Unit failure short-circuits before
paying the ~150 MB Garage image pull.

Caches via dogfooded cacher:
* Go tarball — `cacher download --url` fallback, same pattern as
  publish.yml.
* Garage docker image — `cacher download` of the tar; on miss does
  docker pull + save + upload to seed.

Also silences testcontainers' default logger in the garage testutil
(pull progress, container IDs, port mappings); test output now only
shows what the tests print themselves. `-v` dropped from the e2e
`go test` since per-test progress is no longer drowned out and not
worth printing on green runs.

`TESTCONTAINERS_RYUK_DISABLED=true` — VM is torn down at job end,
reaper container is just startup noise.

Verified end-to-end on job #153 (all 8 tasks ✔, cache MISS seeded).

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).