Pin all images by sha256 digest and restructure for the standard
split-stage pattern.
- Lint stage: golangci/golangci-lint:v1.64.8 pinned by sha256
- Test stage: golang:1.22 pinned by sha256, depends on lint via COPY
- Final stage: golang:1.22 pinned by sha256
- COPY --from=lint forces BuildKit to execute the lint stage
