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>

tests/test_issue_37_emoji_flag_basic.py

@@ -0,0 +1,175 @@
+"""
+Test basic emoji flag functionality - Issue #37
+
+Tests the implementation of --emoji flag and MARKITECT_EMOJI environment variable
+to complement the existing --ascii flag functionality.
+"""
+
+import pytest
+import os
+import subprocess
+import sys
+from pathlib import Path
+from unittest.mock import patch, mock_open
+
+
+class TestEmojiFlag:
+    """Test emoji flag functionality in CLI tools."""
+
+    def test_emoji_flag_exists_in_visualize_schema(self):
+        """Test that --emoji flag is available in visualize_schema.py - Issue #37."""
+        # Test that the --emoji flag is recognized by the argument parser
+        result = subprocess.run([
+            sys.executable, 'tools/visualize_schema.py', '--help'
+        ], capture_output=True, text=True)
+
+        assert result.returncode == 0
+        assert "--emoji" in result.stdout
+        assert "Use emoji output" in result.stdout or "Enable emoji output" in result.stdout
+
+    def test_emoji_flag_exists_in_schema_summary(self):
+        """Test that --emoji flag is available in schema_summary.py - Issue #37."""
+        result = subprocess.run([
+            sys.executable, 'tools/schema_summary.py', '--help'
+        ], capture_output=True, text=True)
+
+        assert result.returncode == 0
+        assert "--emoji" in result.stdout
+        assert "Use emoji output" in result.stdout or "Enable emoji output" in result.stdout
+
+    def test_emoji_flag_mutually_exclusive_with_ascii_visualize_schema(self):
+        """Test that --emoji and --ascii flags are mutually exclusive in visualize_schema - Issue #37."""
+        # Create a temporary test file
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            result = subprocess.run([
+                sys.executable, 'tools/visualize_schema.py', str(test_file), '--ascii', '--emoji'
+            ], capture_output=True, text=True)
+
+            # Should fail with argument parsing error due to mutual exclusivity
+            assert result.returncode == 2, "Using both --ascii and --emoji should result in argument parsing error"
+            assert "not allowed with argument" in result.stderr, "Error message should indicate mutual exclusivity"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_emoji_flag_mutually_exclusive_with_ascii_schema_summary(self):
+        """Test that --emoji and --ascii flags are mutually exclusive in schema_summary - Issue #37."""
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            result = subprocess.run([
+                sys.executable, 'tools/schema_summary.py', str(test_file), '--ascii', '--emoji'
+            ], capture_output=True, text=True)
+
+            # Should fail with argument parsing error due to mutual exclusivity
+            assert result.returncode == 2, "Using both --ascii and --emoji should result in argument parsing error"
+            assert "not allowed with argument" in result.stderr, "Error message should indicate mutual exclusivity"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_emoji_output_is_default_in_visualize_schema(self):
+        """Test that emoji output is the default behavior in visualize_schema - Issue #37."""
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            # Test default output (should have emojis)
+            result_default = subprocess.run([
+                sys.executable, 'tools/visualize_schema.py', str(test_file)
+            ], capture_output=True, text=True)
+
+            # Test explicit emoji flag
+            result_emoji = subprocess.run([
+                sys.executable, 'tools/visualize_schema.py', str(test_file), '--emoji'
+            ], capture_output=True, text=True)
+
+            assert result_default.returncode == 0
+            assert result_emoji.returncode == 0
+
+            # Both should contain emoji characters (basic check)
+            default_has_emojis = any(ord(char) > 127 for char in result_default.stdout)
+            emoji_has_emojis = any(ord(char) > 127 for char in result_emoji.stdout)
+
+            assert default_has_emojis, "Default output should contain emoji characters"
+            assert emoji_has_emojis, "Explicit --emoji flag should produce emoji output"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_emoji_output_is_default_in_schema_summary(self):
+        """Test that emoji output is the default behavior in schema_summary - Issue #37."""
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            # Test default output (should have emojis)
+            result_default = subprocess.run([
+                sys.executable, 'tools/schema_summary.py', str(test_file)
+            ], capture_output=True, text=True)
+
+            # Test explicit emoji flag
+            result_emoji = subprocess.run([
+                sys.executable, 'tools/schema_summary.py', str(test_file), '--emoji'
+            ], capture_output=True, text=True)
+
+            assert result_default.returncode == 0
+            assert result_emoji.returncode == 0
+
+            # Both should contain emoji characters (basic check)
+            default_has_emojis = any(ord(char) > 127 for char in result_default.stdout)
+            emoji_has_emojis = any(ord(char) > 127 for char in result_emoji.stdout)
+
+            assert default_has_emojis, "Default output should contain emoji characters"
+            assert emoji_has_emojis, "Explicit --emoji flag should produce emoji output"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_ascii_flag_overrides_emoji_default_visualize_schema(self):
+        """Test that --ascii flag overrides emoji default in visualize_schema - Issue #37."""
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            result = subprocess.run([
+                sys.executable, 'tools/visualize_schema.py', str(test_file), '--ascii'
+            ], capture_output=True, text=True)
+
+            assert result.returncode == 0
+
+            # Should NOT contain emoji characters
+            has_emojis = any(ord(char) > 127 for char in result.stdout)
+            assert not has_emojis, "ASCII mode should not contain emoji characters"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_ascii_flag_overrides_emoji_default_schema_summary(self):
+        """Test that --ascii flag overrides emoji default in schema_summary - Issue #37."""
+        test_file = Path("temp_test_schema.json")
+        test_file.write_text('{"type": "object", "properties": {"name": {"type": "string"}}}')
+
+        try:
+            result = subprocess.run([
+                sys.executable, 'tools/schema_summary.py', str(test_file), '--ascii'
+            ], capture_output=True, text=True)
+
+            assert result.returncode == 0
+
+            # Should NOT contain emoji characters
+            has_emojis = any(ord(char) > 127 for char in result.stdout)
+            assert not has_emojis, "ASCII mode should not contain emoji characters"
+
+        finally:
+            if test_file.exists():
+                test_file.unlink()