Add PHPDoc and PHPUnit test skills with comprehensive guidelines

- Introduced `phpdoc-code-style` skill for normalizing PHP files and PHPDoc comments according to repository conventions.
- Added detailed documentation for PHPDoc code style, including workflow, defaults, output contract, and anti-patterns.
- Created `phpunit-tests` skill to generate and manage PHPUnit tests while adhering to local conventions and Prophecy usage.
- Included extensive guidelines for writing tests, project discovery, and generation checklists.
- Developed `sphinx-docs` skill for generating and maintaining Sphinx documentation, with a structured approach to documentation layout and content.
- Established reference materials for reStructuredText formatting, documentation structure, and update triggers.
- Updated AGENTS.md to reflect new skills and their usage in the development workflow.

Signed-off-by: Felipe Sayão Lobato Abreu <github@mentordosnerds.com>

Author: coisa