Full-Chain Validation Engine

[djm81]

Why

Full-chain validation should surface clean-code results without redefining the traceability graph.

What Changes

• Keep Req -> Arch -> Spec -> Code -> Tests as the only chain layers.
• Add optional --with-code-quality support so specfact review can contribute a side-channel summary.
• Pass the clean-code summary into the governance evidence envelope as code_quality.

OpenSpec Change Proposal: validation-02-full-chain-engine

[djm81]

Dependency mapping from CHANGE_ORDER: blocked by #239, #240, #176.