Add CORTEX - AI Agent OS with persistent memory, Ed25519 identity and dream mode

[ThankNIXlater]

Description

This PR adds CORTEX to the open-source AI agents list.

What is CORTEX?

CORTEX is an AI Agent Operating System built with original architecture (not a fork of any existing project). It solves the core problem most AI agent frameworks ignore: agents forget their identity and state across sessions.

Key features:

• Persistent memory - Structured memory that survives restarts, crashes, and model swaps
• Ed25519 cryptographic identity - Every agent action is signed and verifiable
• Dream mode - Background reasoning during idle periods; agent consolidates knowledge while offline
• Multi-model support - Provider-agnostic routing across Claude, GPT-4o, Gemini, Llama, and others
• Multi-agent coordination - Built-in agent bus for orchestrating multiple instances

Why it belongs here:
CORTEX is a fully open-source autonomous agent OS. It fits the "General purpose / Build your own / Multi-agent" category alongside similar entries like Adala, AgentBench, and CAMEL.

Links:

• GitHub: https://github.com/ThankNIXlater/cortex
• Website: https://nixus.pro/cortex/

Added in alphabetical order after Continue and before CrewAI.