Contributor task: Add documentation for contributing and local development

[ashishsinghbora]

Documentation task

Make Magicloder approachable to external contributors.

Requirements

• Explain the architecture and repository structure.
• Explain how to run the daemon, CLI, tests and frontend locally.
• Explain how an issue should be selected and how PRs should be structured.
• Document coding conventions and required checks.
• Add a contributor troubleshooting section.
• Link to the architecture and API documentation.

Done when

A new contributor can get the project running from a clean machine using only the repository documentation.