feat(pf-design-tokens): add analyze-modifiers skill

[srambach]

*Note: I believe this belongs in a new plugin for core/CSS, not in the design-tokens plugin, but since one doesn't exist I'll put it here for now.

Summary

• Adds a new skill to discover, analyze, and document PatternFly modifier classes (pf-m-* classes)
• Searches through component SCSS files to extract all modifier usage
• Generates comprehensive markdown reports organized by modifier or by component
• Provides usage statistics and identifies patterns across the design system

Use Cases

• Document all available modifiers for a component
• Find where a specific modifier is used across components
• Audit modifier patterns and identify inconsistencies
• Generate modifier reference documentation

Test plan

• Run /analyze-modifiers on the PatternFly codebase
• Test component-specific analysis: /analyze-modifiers for Button
• Test modifier-specific search: /analyze-modifiers find pf-m-disabled
• Verify output includes responsive variants and nested modifiers
• Check that generated markdown files are properly formatted

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