feat: implement --emoji flag and MARKITECT_EMOJI environment variable - Issue #37

Add comprehensive emoji preference support to complement existing --ascii flag:

🎯 Core Features:
• Add --emoji flag to visualization tools (visualize_schema.py, schema_summary.py)
• Implement MARKITECT_EMOJI environment variable support
• Maintain backward compatibility with existing --ascii flag behavior
• Establish proper priority: CLI flags > environment variables > defaults

🏗️ Architecture:
• Create shared emoji_utils.py module for centralized logic
• Implement determine_output_mode() for standardized preference resolution
• Add add_emoji_arguments() for consistent argument parser setup
• Follow DRY principle - eliminate duplicate code between tools

🧪 Testing:
• 18 comprehensive tests covering all functionality
• Basic flag tests: existence, mutual exclusivity, defaults, precedence
• Environment variable tests: recognition, case handling, CLI overrides
• Configuration integration tests: system compatibility, error handling
• All 1337 project tests pass (no regressions)

💡 User Experience:
• Consistent behavior across all MarkiTect visualization tools
• Multiple preference setting methods (CLI flags, environment variables)
• Robust error handling with sensible defaults (emoji by default)
• Clear help documentation and discoverable usage patterns

🔧 Implementation Details:
• Mutually exclusive argument groups prevent conflicting flags
• Case-insensitive environment variable processing
• Valid false values: 'false', 'f', '0' - all others default to emoji
• Comprehensive documentation with usage examples

The implementation follows TDD principles and MarkiTect architectural
patterns, ensuring high quality and maintainability while delivering
enhanced usability features.

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

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

tools/visualize_schema.py

@@ -6,12 +6,15 @@ Beautiful command-line visualization for markdown schema structure.
 import sys
 import json
 import argparse
+import os
 from pathlib import Path
 
 # Add markitect to path
 sys.path.insert(0, '.')
 
 from markitect.schema_generator import SchemaGenerator
+# Issue #37: Import shared emoji/ASCII output mode utilities
+from emoji_utils import determine_output_mode, add_emoji_arguments
 
 def visualize_schema_structure(file_path, max_depth=None, ascii_only=False):
     """Create a beautiful tree visualization of the document structure."""
@@ -177,7 +180,9 @@ def main():
     parser = argparse.ArgumentParser(description='Visualize markdown document schema structure')
     parser.add_argument('file_path', help='Path to the markdown file')
     parser.add_argument('--max-depth', type=int, help='Maximum heading depth to include')
-    parser.add_argument('--ascii', action='store_true', help='Use ASCII characters only (no colorful icons)')
+
+    # Issue #37: Add emoji/ASCII output format arguments
+    add_emoji_arguments(parser)
 
     args = parser.parse_args()
 
@@ -185,7 +190,11 @@ def main():
         print(f"File not found: {args.file_path}")
         sys.exit(1)
 
-    visualize_schema_structure(args.file_path, args.max_depth, args.ascii)
+    # Issue #37: Determine output mode using shared utility
+    # Respects CLI flags and MARKITECT_EMOJI environment variable
+    use_ascii = determine_output_mode(args)
+
+    visualize_schema_structure(args.file_path, args.max_depth, use_ascii)
 
 if __name__ == "__main__":
     main()