feat: add consumer agent guardrails

[fastner]

Summary

• add a standards-owned Consumer AGENTS section for managed-file guardrails
• sync the section through manifest sections and include CONSUMER-AGENTS.md in the package
• clarify oxfmt guidance in changes/0005 and add changes/0007 for standards version 7
• make marker sections formatter-stable with a blank line after the start marker

Validation

• pnpm vitest run test/standards.test.ts
• pnpm agent:check

Plan: docs/plan/0016-consumer-agents-oxfmt-guidance.md