docs(design): Add DeepAgent + DocumentAgent integration design

Add comprehensive design documentation for integrating DocumentAgent family
into DeepAgent as LangChain tools:

- deepagent_document_tools_integration.md (1,500+ lines):
  * Complete architecture analysis and design strategy
  * Three-tier tool hierarchy (Basic → AI → Smart)
  * Detailed tool specifications with code examples
  * Integration patterns and execution flows
  * Quality assurance (99-100% accuracy preservation)
  * Performance optimizations (caching, async, streaming)
  * Security measures and resource limits
  * Testing strategy (unit, integration, E2E)
  * 4-phase deployment plan (8 weeks)

- integration_architecture_summary.md (600+ lines):
  * Visual architecture diagrams (ASCII art)
  * Quick reference for developers
  * Tool selection flowcharts
  * Interaction examples
  * Configuration templates
  * Benefits summary

Design enables:
✓ Natural language document processing via DeepAgent
✓ Automatic tool selection based on user intent
✓ Multi-turn conversations about documents
✓ Quality preservation (all DocumentAgent features)
✓ Graceful fallback chain (Smart → AI → Basic)
✓ Session-aware context management

Related to: DocumentAgent (requirements extraction), AIDocumentAgent
(semantic analysis), TagAwareDocumentAgent (context-aware processing)

Author: vinod0m