docs: comprehensive Codex feature analysis and recommendations update (#176)

- Cloned and analyzed OpenAI Codex repository (v0.90.0-alpha)
- Added detailed comparison table: Codex vs Cortex-CLI
- Documented new Codex features not in cortex-cli:
  - Network Proxy with SSRF protection and domain filtering
  - Multi-Agent Collaboration system (spawn_agent, send_input, wait, close_agent)
  - Shell Snapshot System for shell state persistence
  - ThreadManager for centralized session management
  - TUI Backtracking for conversation history navigation
  - Advanced Feature Flags system with stages
  - Collaboration Modes (Plan, Code, PairProgramming, Execute)
  - Starlark Execution Policy engine
- Updated implementation status for existing features:
  - Seatbelt sandbox (already implemented in cortex)
  - Ollama/LMStudio clients (already implemented)
  - Ghost/Undo system (already implemented)
- Added detailed code samples from Codex source
- Added prioritized implementation roadmap
- Added architecture diagrams for integration

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt