docs: Add orchestrator-quota separation architecture and implementation guide

- Add ORCHESTRATOR_QUOTA_SEPARATION.md with three-layer architecture design
  - Layer 1: Orchestrator (platform policy)
  - Layer 2: Domain Services (business logic)
  - Layer 3: Infrastructure (shared utilities)
- Add comprehensive table of contents with anchor links
- Include information flow diagram, cost calculation examples
- Document extensibility patterns and separation of concerns benefits

- Add ORCHESTRATOR_QUOTA_SEPARATION_IMPLEMENTATION.md with detailed guide
  - 8 implementation steps with exact code locations
  - Complete quotaTracker.js module with tests
  - Call tracking updates for geminiClient
  - Cost calculation and quota check logic
  - 202 deferral response handling
  - Testing strategy and error handling scenarios
  - Verification checklist (25+ items)
  - Rollback plan for clean reversal if needed

Status: Ready for engineer implementation (2-3 hour effort)

Author: klavay