v1.4.0

Three Go-focused components:

- go-build: branch verification cross-compile (matrix of
  GOOS/GOARCH, version-from-ref ldflags). Skipped on tags.
- go-release-binary: tag-only build, sha256 checksums, cosign
  keyless signing via GitLab OIDC, upload to the project's
  generic package registry.
- release-create: tag-only GitLab Release creation with annotated
  tag message + asset linking from the package registry.

Together these enable a full branch-and-tag CI shape for any Go
project in the dunn.dev estate. First consumer: dunn.dev/bairn.

Modeled on the binary + release templates in
gitlab.com/gitlab-com/public-sector-tools/pipeline (Apache 2),
scaled down to the dunn.dev estate's needs (no SBOM, no SLSA
attestation; those layer on later if a project needs them).

Existing components (claude-drift-triage, cloudflare-pages,
cloudflare-r2, cloudflare-worker, container-image) ship at v1.4.0
unchanged.