docs: Add agentic RAG architecture documentation (#701)

Add comprehensive architecture documentation for the Agentic RAG Platform:

- agentic-ui-architecture.md: React component hierarchy, state management,
  and API integration for agent features
- backend-architecture-diagram.md: Overall backend architecture with
  Mermaid diagrams showing service layers and data flow
- mcp-integration-architecture.md: MCP client/server integration strategy,
  PR comparison (#671 vs #684), and Context Forge integration
- rag-modulo-mcp-server-architecture.md: Exposing RAG capabilities as MCP
  server with tools (rag_search, rag_ingest, etc.) and resources
- search-agent-hooks-architecture.md: 3-stage agent pipeline (pre-search,
  post-search, response) with database schema and execution flow
- system-architecture.md: Complete system architecture overview with
  technology stack and data flows

These documents guide implementation of:
- PR #695 (SPIFFE/SPIRE agent identity)
- PR #671 (MCP Gateway client)
- Issue #697 (Agent execution hooks)
- Issue #698 (MCP Server)
- Issue #699 (Agentic UI)

Closes #696

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Co-authored-by: Claude <noreply@anthropic.com>

Author: manavgup