ci: unified docker workflow (shadow mode)

[t0mdavid-m]

Summary

• Adds .github/workflows/build-and-test.yml — unified matrix build (full + simple) with registry cache, kind integration, manifest lint.
• Adds .github/workflows/ghcr-cleanup.yml — scheduled retention policy (schedule disabled; dry-run default true until Phase 4).
• Old workflows (build-docker-images.yml, build-and-push-image.yml, k8s-manifests-ci.yml) remain active. Shadow mode — new and old run in parallel.

Test plan

• Merge this PR (no tags change yet — safe).
• On the first push to main after merge, confirm new workflow succeeds end-to-end.
• Inspect GHCR: main-full, main-simple, <sha>-full, <sha>-simple, latest all appear. cache:full and cache:simple appear under the /cache package.
• Push a trivial follow-up commit; confirm full-variant rebuild takes minutes, not hours (registry cache working).
• Manually trigger ghcr-cleanup.yml with dry-run: true and inspect output.

Linked spec: docs/superpowers/specs/2026-04-20-unified-docker-ci-design.md (local to author — not checked in; docs/superpowers/ is gitignored).

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: f24edd81-5a9e-4473-9403-8a3c1e162b88

📥 Commits

Reviewing files that changed from the base of the PR and between eae014b and e7f6c08.

📒 Files selected for processing (2)

• .github/workflows/build-and-test.yml
• .github/workflows/ghcr-cleanup.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/unify-docker-workflows

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