docs(release): add v1 readiness gate

[stacknil]

Summary

• add docs/v1-readiness-gate.md defining v1.0 fixed inputs, fixed outputs, schema validation, artifact regeneration, and test pass requirements
• link the gate from README, docs index, reviewer pack, roadmap, and v1 contract freeze docs
• add reviewer-doc regression coverage for the new gate and links

Validation

• pytest tests/test_reviewer_docs.py tests/test_markdown_links.py
• python scripts/regenerate_artifacts.py --check
• pytest
• git diff --check

Safety

• docs/test-only change
• no new demo surface
• privacy scan on touched files found no account IDs, AWS key prefixes, local paths, or local usernames