docs: add comprehensive Codex feature recommendations for cortex-cli (#175)

This document analyzes OpenAI Codex codebase and provides detailed
technical documentation of features that could be ported to cortex-cli:

- macOS Seatbelt sandbox implementation (critical)
- Network proxy with policy enforcement
- Multi-agent collaboration system (spawn_agent, send_input, wait, close_agent)
- Ollama and LMStudio local model integrations
- Shell snapshot system for environment preservation
- ThreadManager for advanced session management
- Ghost snapshot (undo) system
- Advanced TUI features (backtracking, pager overlay, resume picker)

Includes implementation priorities, architecture diagrams, and code examples
from the Codex codebase.

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

Author: echobt