feat(policy): canonical YAML policy schema + examples (2.3a)

[Shaivpidadi]

Summary

• add the canonical policy-as-code JSON Schema for precheck YAML policies
• add minimal, standard, and enterprise example policy files
• validate the schema and examples in pytest, and document the canonical contract in PROJECT_SPECS.md

GovernsAI Tracker issue

GOV-564

Reviewers

Tagging Nexus (code quality) and Cipher (security/arch) — both approvals required.

Test plan

• .venv/bin/pytest tests/ -v