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/schema_summary.py

@@ -5,12 +5,15 @@ Schema summary tool - provides concise 4-line summary of markdown structure.
 
 import sys
 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 generate_summary(file_path, ascii_mode=False):
     """Generate a concise 4-line summary of the document structure."""
@@ -89,13 +92,18 @@ def generate_summary(file_path, ascii_mode=False):
 def main():
     parser = argparse.ArgumentParser(description='Generate concise schema summary')
     parser.add_argument('file_path', help='Path to the markdown file')
-    parser.add_argument('--ascii', action='store_true',
-                       help='Use ASCII characters only (no emojis)')
+
+    # Issue #37: Add emoji/ASCII output format arguments
+    add_emoji_arguments(parser)
 
     args = parser.parse_args()
 
+    # Issue #37: Determine output mode using shared utility
+    # Respects CLI flags and MARKITECT_EMOJI environment variable
+    use_ascii = determine_output_mode(args)
+
     try:
-        summary_lines = generate_summary(args.file_path, args.ascii)
+        summary_lines = generate_summary(args.file_path, use_ascii)
         for line in summary_lines:
             print(line)
     except Exception as e: