Create a CONTRIBUTING.md to explain how to contribute to the project

### Objective
Create a CONTRIBUTING.md file

### Context
This project have a heavy architecture and uses a lot of the last features of IA agents and classic open source management, for that is necessary write the path that a dev can contribute appropriately to the project.

### Checklist
- [ ] Add a section that explain what is the software architecture of the project
- [ ] Add a section that explain how implement code with the TDD philosophy
- [ ] Add a section that explain how write docs with use cases and UML
- [ ] Add a section that explain how we are using OpenCode and what features we have integrated
- [ ] Add a section with good practices of opens ource contributions like: commits, branch names, PR context, code quality
- [ ] Add a section for a guide of code style or conventions to work with nestjs or typescript

Create CONTRIBUTING.md at the repository root. First inspect the project architecture, existing TypeScript/NestJS conventions, testing setup, documentation practices, and OpenCode integrations. Done means the guide covers the requested architecture, TDD, use cases and UML, OpenCode, contribution practices, and code style sections.

Written by the indexing model from the issue text.