feat: implement modular theme system with file-based theme organization

Transform theme system from large inline dictionaries to maintainable YAML files:

**Architecture:**
- File-based themes organized by scope: mode/, ui/, document/, branding/
- Dynamic theme loading with automatic discovery
- Hybrid system maintaining 100% backward compatibility
- Rich metadata support with theme documentation

**Implementation:**
- Created markitect/themes/ directory with organized structure
- Added ThemeRegistry for dynamic YAML theme loading
- Extracted ChatGPT and Substack themes to separate files
- Added mode themes (light.yaml, dark.yaml) as examples
- Integrated with existing LAYERED_THEMES system seamlessly

**Benefits:**
- Improved maintainability: each theme is a separate file
- Better collaboration: multiple contributors can work simultaneously
- Enhanced discoverability: clear organization shows available themes
- Rich documentation: each theme file includes design notes and metadata
- Schema validation potential with YAML format

**Quality Assurance:**
- Comprehensive 12-test suite for modular system (12/12 passing)
- Backward compatibility verified with existing 15 theme tests (15/15 passing)
- CLI integration tested and working with file-based themes
- Theme combination and scoping functionality preserved

**Files Created:**
- markitect/themes/__init__.py - Theme registry and dynamic loader
- markitect/themes/README.md - Complete documentation and usage guide
- markitect/themes/document/{chatgpt,substack}.yaml - Modular theme files
- markitect/themes/mode/{light,dark}.yaml - Mode theme examples
- tests/test_modular_theme_system.py - Comprehensive test coverage

Addresses maintainability concerns while preserving all existing functionality.
No breaking changes - all existing code, CLI commands, and API calls work unchanged.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/themes/document/chatgpt.yaml

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+# ChatGPT Document Theme
+# Mimics ChatGPT's chat interface fonts and layout for compact, interactive reading
+# Issue #165: https://github.com/example/repo/issues/165
+
+name: chatgpt
+description: "Compact, modern theme inspired by ChatGPT's interface design"
+scope: document
+author: "Claude Code"
+version: "1.0.0"
+
+properties:
+  # Typography - Modern sans-serif stack
+  font_family: 'Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif'
+  heading_font_family: 'Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif'
+  font_size: '15px'
+
+  # Layout - Compact for interactive reading
+  max_width: '580px'
+  line_height: '1.5'
+  text_align: 'left'
+
+  # Colors - High contrast with ChatGPT green accents
+  body_background: '#ffffff'
+  body_color: '#1f1f1f'
+  heading_color: '#1f1f1f'
+  accent_color: '#10a37f'
+  link_color: '#10a37f'
+  link_hover_color: '#0d8c6d'
+
+  # Code styling
+  code_background: '#f7f7f7'
+  code_color: '#1f1f1f'
+  code_font_family: '"SF Mono", Monaco, Inconsolata, "Roboto Mono", Consolas, "Courier New", monospace'
+
+  # Spacing - Compact margins for efficiency
+  heading_style: 'minimal'
+  heading_margin: '1.2em 0 0.6em 0'
+  paragraph_margin: '1em 0'
+
+  # Visual elements
+  border_radius: '8px'
+  blockquote_border: '#10a37f'
+  blockquote_color: '#6b7280'
+
+# Design notes:
+# - Inter font provides clean, modern readability
+# - 580px max-width creates chat-like compactness
+# - 1.5 line height balances readability with density
+# - ChatGPT green (#10a37f) creates brand consistency
+# - Minimal margins maximize information density

markitect/themes/document/substack.yaml

@@ -0,0 +1,43 @@
+# Substack Document Theme
+# Mimics Substack's typography and layout for enhanced long-form reading
+# Issue #166: https://github.com/example/repo/issues/166
+
+name: substack
+description: "Elegant theme inspired by Substack's long-form reading experience"
+scope: document
+author: "Claude Code"
+version: "1.0.0"
+
+properties:
+  # Typography - Serif for long-form reading
+  font_family: 'Spectral, Georgia, "Times New Roman", serif'
+  heading_font_family: 'Lora, -apple-system, BlinkMacSystemFont, sans-serif'
+
+  # Layout - Optimized for long-form content
+  max_width: '680px'
+  line_height: '1.6'
+  text_align: 'left'
+
+  # Colors - Warm, cream background for comfort
+  body_background: '#FAF9F1'
+  body_color: '#333333'
+  heading_color: '#333333'
+  accent_color: '#b08d57'
+  link_color: '#b08d57'
+  link_hover_color: '#8b6c42'
+
+  # Code styling
+  code_background: '#f5f4ed'
+  code_color: '#333333'
+
+  # Visual elements
+  heading_style: 'simple'
+  blockquote_border: '#b08d57'
+  blockquote_color: '#666666'
+
+# Design notes:
+# - Spectral serif font optimized for digital long-form reading
+# - 680px width follows optimal line length for comprehension
+# - 1.6 line height provides generous reading comfort
+# - Warm cream background reduces eye strain during long sessions
+# - Bronze accents create sophisticated, literary feel