Add subscription system documentation and enforcement guides

- SUBSCRIPTION_AUDIT_SUMMARY.md: Overview of current state and gaps
- SUBSCRIPTION_PLAN_SYSTEM.md: Complete technical reference
- LIMIT_ENFORCEMENT_IMPLEMENTATION.md: Step-by-step implementation guides
- SUBSCRIPTION_QUICK_REFERENCE.md: API cheat sheet
- SUBSCRIPTION_DOCS_INDEX.md: Navigation and quick start

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: etewiah