feat(policy): add fail-closed declarative admission mutations

[scttbnsn]

Why\n\nSockguard already rewrites owner labels and verified image references internally, while mikesir87/docker-socket-proxy exposes useful operator-configurable request mutators. A bounded mutation layer can enforce mandatory metadata and controlled image migration without giving plugins arbitrary code execution.\n\n## Initial scope\n\n- Mandatory label injection for supported create surfaces.\n- Exact/prefix image-reference remapping for container and service deployment.\n- Deterministic ordering relative to owner stamping and image-trust digest pinning.\n- Canonicalize and fully re-inspect the mutated body before forwarding.\n- Audit both the mutation rule identifier and final policy verdict without logging secret values.\n\n## Acceptance\n\n- A mutation cannot introduce a field that bypasses body, ownership, or image-trust policy.\n- Case-variant/duplicate JSON protections still hold after rewriting.\n- Warn/audit rollout modes describe the would-mutate result without mutating forwarded traffic.\n- Mutation is bounded declarative configuration, not an arbitrary execution hook.