docs: require pull requests for non-release changes

[minixalpha]

Summary

• require agents to create or switch to a dedicated branch before modifying repository files
• require every non-release change to reach main through a pull request and the land-pr confirmation gate
• retain the explicitly requested release workflow as the sole exception for direct commits and pushes to main

Validation

• git diff --check
• CI: pytest (Python 3.13)
• CI: pytest (Python 3.14)