docs: add instruction-quality pre-commit gate

[roli-lpci]

Summary

• document an optional deterministic LintLang pre-commit gate for version-controlled agent instructions
• keep the example limited to AGENTS.md and explain how to select only existing Claude Code, Copilot, or Gemini instruction paths
• preserve the advisory baseline and link the existing CI/SARIF initializer

Validation

• npm run lint (passes; one pre-existing unused-variable warning)
• npm run typecheck
• npm test (57 passing)
• npm run build

This is documentation only: it uses LintLang’s native pre-commit/CI interfaces and adds no agent-specific adapter.