Add project: Fellow

[ManuelZierl]

What kind of change does this PR introduce?

• [x] Add a project
• [ ] Update a project
• [ ] Remove a project
• [ ] Add or update a category
• [ ] Change configuration
• [ ] Documentation
• [ ] Other, please describe:

Description: Added Fellow Fellow is a command-line AI assistant built by developers, for developers.

Unlike most AI tools that stop at suggesting code, Fellow goes a step further: It executes tasks on your behalf. It reasons step-by-step, chooses commands from a plugin system, and edits files, generates content, or writes tests — autonomously.

Checklist:

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have not modified the README.md file. Projects are only supposed to be added or updated within the projects.yaml file since the README.md file is automatically generated.