Refactor skills tool integration and enhance system prompt

- Removed the skills_categories tool from the skills toolset, streamlining the skills functionality to focus on skills_list and skill_view.
- Updated the system prompt to dynamically build a compact skills index, allowing the model to quickly reference available skills without additional tool calls.
- Cleaned up related code and documentation to reflect the removal of skills_categories, ensuring clarity and consistency across the codebase.

tools/__init__.py

@@ -59,7 +59,6 @@ from .image_generation_tool import (
 )
 
 from .skills_tool import (
-    skills_categories,
     skills_list,
     skill_view,
     check_skills_requirements,
@@ -158,7 +157,6 @@ __all__ = [
     'image_generate_tool',
     'check_image_generation_requirements',
     # Skills tools
-    'skills_categories',
     'skills_list',
     'skill_view',
     'check_skills_requirements',