feat: Refactor Cortex Monorepo

[louis-jan]

Problem

💡 We agreed that a single monorepo could offer the following benefits:

1. Maintainability - Every Cortex engineer would be able to perform code reviews on every PR. Easy to test, cortex-cpp binaries are always being updated.
2. Single submodule - Jan's users would only need to know one Cortex, reducing confusion caused by separate repositories. Even if we were to create two separate repositories, Jan's repository would simply add one CortexJS submodule. CortexJS would still exclusively reference Cortex CPP binaries.

janhq/cortex
│
├── cortex-js/               # Nest.js backend (NestJS)
│   ├── package.json         # Backend project dependencies
│   ├── README.md
│   ├── Dockerfile
│   ├── docker-compose.yml
│   ├── src/                 # Backend source code
│   │   ├── controllers/     # Nest.js controllers
│   │   ├── modules/         # Nest.js modules
│   │   ├── services/        # Nest.js services
│   │   └── ...              # Other Nest.js backend-related files
│   └── ...                  # Other Nest.js backend-related files
│
├── cortex-cpp/              # C++ backend with DrogonFramework
│   ├── app/                 # Backend source code
│   │   ├── controllers/     # Drogon controllers
│   │   ├── models/          # Data models
│   │   ├── services/        # Business logic services
│   │   ├── ?engines/        # Inference Engine submodules
│   │   |   ├──llama.cpp
│   │   |   ├──tensorrt-llm
│   │   |   └── ...
│   │   └── ...              # Other Drogon backend-related files
│   │
│   ├── CMakeLists.txt       # CMake build configuration
│   ├── config.json          # Drogon configuration
│   ├── Dockerfile
│   ├── docker-compose.yml
│   ├── README.md
│   └── ...                  # Other Drogon backend-related files
│
├── scripts/                     # Utility scripts
│   └── ...
├── README.md
├── package.json
├── Dockerfile					         # Dockerfile to deploy both
├── docker-compose.yml			     # Docker compose file
└── docs/                        # Documentation files
    └── ...

Success Criteria
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Additional context
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[louis-jan]

@namchuai added cortex-js here janhq/jan#530
@tikikun please proceed for the cortex-cpp
@hiento09 would help adding CI/scripts files

[louis-jan]

It's completed