ci: use shared ghcr workflow from init4tech/actions

[rswanson]

[Claude Code]

Summary

• Replaces the local 128-line GHCR workflow with a minimal caller to the shared reusable workflow at init4tech/actions/.github/workflows/ghcr.yml@main
• Same triggers preserved (release: [published] + workflow_dispatch)
• Gains artifact attestation from the shared workflow

Behavioral changes

• Runner group changes from amd64-large-runners/arm64-large-runners to init4-runners
• GHA build caching removed (shared workflow doesn't use it)
• type=semver,pattern={{major}} tag no longer generated
• latest tag applies on every trigger (fine since triggers are release + dispatch only)
• Artifact attestation added via actions/attest-build-provenance@v3

Test plan

• Trigger a manual workflow_dispatch run from the Actions tab
• Confirm both linux/amd64 and linux/arm64 images are built and pushed to GHCR
• Verify attestation artifacts appear on the package

🤖 Generated with Claude Code