The image now bakes only TorchLean as the shared prebuilt store (PKC is
not yet open source and cannot ship). A CUDA-invariant layer clones
TorchLean and runs `lake exe cache get`, so Mathlib is downloaded once
and shared across :cpu/:gpu; package-overrides.json is generated from
TorchLean's manifest. Removed the PKC clone/build, its BuildKit token
secret, and every PKC reference across the Dockerfile, README,
gitlab-ci.yaml, run-image.sh, and verify-dedup.sh.

Add OCI image metadata (org.opencontainers.image.* + custom
org.maap.soil-moisture.*): title/description/authors/vendor/licenses/url,
source/revision/version/created, base.name/base.digest, and
lean-toolchain/torchlean-ref/torchlean-revision/cuda-enabled. Declared as
the final layer so a changing SHA/date only rebuilds trivial metadata.
build-images.sh computes and passes these (git SHA, RFC-3339 date, base
digest, resolved TorchLean commit) and now refuses to build a non-git or
dirty tree so the recorded revision matches the source. Add LICENSE
(Apache-2.0).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>