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/mode/dark.yaml

@@ -0,0 +1,36 @@
+# Dark Mode Theme
+# Dark color scheme for low-light reading
+
+name: dark
+description: "Comfortable dark color scheme for low-light environments"
+scope: mode
+author: "Markitect Core"
+version: "1.0.0"
+
+properties:
+  # Base colors
+  body_background: '#0d1117'
+  body_color: '#e6edf3'
+  heading_color: '#f0f6fc'
+
+  # Code blocks
+  code_background: '#161b22'
+  code_color: '#e6edf3'
+
+  # Borders and lines
+  border_color: '#30363d'
+  table_border: '#30363d'
+  table_header_bg: '#161b22'
+
+  # Blockquotes
+  blockquote_border: '#30363d'
+  blockquote_color: '#7d8590'
+
+  # Links
+  link_color: '#58a6ff'
+  link_hover_color: '#79c0ff'
+
+# Design notes:
+# - GitHub dark theme inspired colors
+# - Reduced eye strain for nighttime reading
+# - Blue links provide good contrast on dark background

markitect/themes/mode/light.yaml

@@ -0,0 +1,36 @@
+# Light Mode Theme
+# Light color scheme for daytime reading
+
+name: light
+description: "Clean light color scheme for comfortable daytime reading"
+scope: mode
+author: "Markitect Core"
+version: "1.0.0"
+
+properties:
+  # Base colors
+  body_background: '#ffffff'
+  body_color: '#333333'
+  heading_color: '#24292f'
+
+  # Code blocks
+  code_background: '#f6f8fa'
+  code_color: '#24292e'
+
+  # Borders and lines
+  border_color: '#d0d7de'
+  table_border: '#d0d7de'
+  table_header_bg: '#f6f8fa'
+
+  # Blockquotes
+  blockquote_border: '#dfe2e5'
+  blockquote_color: '#6a737d'
+
+  # Links
+  link_color: '#0969da'
+  link_hover_color: '#0550ae'
+
+# Design notes:
+# - High contrast for excellent readability
+# - GitHub-inspired color palette for familiarity
+# - Subtle grays for secondary elements