refactor(build): unify image build graph for cache reuse

[drew]

Summary

Simplify the two-image container build pipeline around one shared Docker build graph so gateway and cluster builds reuse the same Rust dependency cache instead of recompiling overlapping workspace state separately.

Related Issue

N/A

Changes

• replace the split gateway and cluster Dockerfiles with a shared deploy/docker/Dockerfile.images build graph
• add a shared docker-build-image.sh helper and route local, publish, and fast-deploy flows through it
• make supervisor fast deploy build from the shared supervisor-builder target so it reuses the same cache path as full image builds
• update architecture docs and the cluster debug skill to reflect the new shared image build layout

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Additional validation:

• bash -n tasks/scripts/docker-build-image.sh tasks/scripts/docker-build-component.sh tasks/scripts/docker-build-cluster.sh tasks/scripts/docker-publish-multiarch.sh tasks/scripts/cluster-deploy-fast.sh tasks/scripts/cluster-bootstrap.sh tasks/scripts/cluster-push-component.sh
• RUSTC_WRAPPER= cargo check --workspace
• docker buildx build --check -f deploy/docker/Dockerfile.images --target gateway .
• docker buildx build --check -f deploy/docker/Dockerfile.images --target cluster .
• docker buildx build --check -f deploy/docker/Dockerfile.images --target supervisor-builder .
• mise run pre-commit (fails on existing workspace lint warnings outside this change)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)