feat: add standard testing scaffolding to python_skill template (#19)

[Polimixanos]

[Feat]: Add standard testing scaffolding to the python_skill template

Description

This PR implements the standard testing scaffolding for the python_skill template as requested in Issue #19. By providing a pre-configured test_skill.py in the boilerplate, we encourage Test-Driven Development (TDD) for all future community skill contributions.

Key Changes:

• Testing Scaffolding: Added templates/python_skill/test_skill.py. This provides a pytest stub that validates skill execution against the output schema defined in manifest.yaml.
• Manifest Standardization: Updated the manifest.yaml template to include parameters (JSON Schema), constitution, and requirements fields, aligning it with loader.py and current framework standards.
• Boilerplate Completion: Added a boilerplate instructions.md to the template and refined skill.py to ensure all mandatory files are present and follow best practices.
• Documentation & Process:
  • Updated CONTRIBUTING.md to mandate test_skill.py for all new skills.
  • Updated README.md architecture diagram.
  • Updated docs/TESTING.md with instructions for running skill-local tests.

Type of Change

• 🧠 Framework Feature / RFC Updates: Core Framework Update (Templates and Contribution Standards)

Checklist

• My code follows the Agent Code of Conduct.
• I have included a properly formatted manifest.yaml in the template.
• The skill logic operates purely in Python and does not rely on arbitrary LLM code generation.
• Requirements and env_vars are explicitly documented in the manifest.
• I have written unit tests proving deterministic execution and schema compliance.
• I have verified that SkillLoader successfully loads this module without missing dependency errors.

Related Issues

Fixes #19