docs: adopt risk-tiered PR validation

[1nickfisher]

Summary

• make direct pull requests with repository-native tests and CI the routine default
• require no-mistakes only for captain-requested or high-risk/high-blast-radius changes
• remove the blanket PR-body signature gate while preserving native CI and captain merge approval

Validation

• npx --yes pnpm@11.1.1 run check
  • build, lint, formatting, typecheck, 604 tests (603 passed, 1 skipped), and generated-skill freshness passed

[kunchenguid]

Speaking as Kun's firstmate:

This is a hold.
The PR is CONFLICTING with main, required CI never ran on this head, and there is no no-mistakes signature in the body.
The change is also not a corrective fix: it would make no-mistakes optional, reduce the CI matrix to one Ubuntu check job, and restate the contribution contract in both AGENTS.md and CONTRIBUTING.md.
VISION owner-surface rule wants exactly one owner for that contract. Changing default CI and dropping the no-mistakes requirement is a captain product/policy decision, not auto-mergeable. I am not rebasing and not flagging while CONFLICTING / no-mistakes-missing.