feat: Complete Issue #52 - Capture actual heading text in schemas

Implement comprehensive heading text capture functionality that allows schemas to
enforce specific heading text requirements through enum constraints:

• New CLI option: --capture-heading-text flag for exact text constraints
• Schema generation with heading text as enum constraints (not just structure)
• Advanced validation engine that enforces heading text requirements
• Metaschema extension: x-markitect-heading-text-capture marker
• Full integration with Issue #51 outline mode capabilities
• Comprehensive error reporting for heading text mismatches
• Complete backward compatibility with existing schema generation

Technical implementation:
- Extended SchemaGenerator with capture_heading_text parameter
- Enhanced validation system to check enum constraints on heading content
- Added _validate_heading_text_constraints_with_errors for detailed reporting
- Integrated with existing metaschema validation from Issue #50
- Preserved document order of headings in enum constraints

Key features:
- Schemas can now specify required heading text via enum constraints
- Validation rejects documents with incorrect heading text
- Detailed error messages show expected vs actual heading text
- Works seamlessly with outline mode depth controls
- Maintains 100% compatibility with 513 existing tests

Usage examples:
  markitect schema-generate --capture-heading-text document.md
  markitect schema-generate --mode outline --capture-heading-text --depth 2 document.md

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

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

markitect/cli.py

@@ -1454,8 +1454,9 @@ def ast_stats(config, file_path, format):
 @click.option('--format', 'output_format', type=click.Choice(['json', 'yaml']), default='json', help='Output format')
 @click.option('--mode', type=click.Choice(['outline']), help='Generation mode: outline for structure-focused schemas')
 @click.option('--depth', type=int, help='Maximum depth for outline mode (similar to --max-depth)')
+@click.option('--capture-heading-text', is_flag=True, help='Capture exact heading text as schema constraints')
 @pass_config
-def generate_schema(config, file_path, max_depth, output, outfile, output_format, mode, depth):
+def generate_schema(config, file_path, max_depth, output, outfile, output_format, mode, depth, capture_heading_text):
     """
     Generate a JSON schema from a markdown file's AST structure.
 
@@ -1470,9 +1471,17 @@ def generate_schema(config, file_path, max_depth, output, outfile, output_format
         markitect schema-generate --mode outline document.md
         markitect schema-generate --mode outline --depth 3 --outfile schema.json document.md
 
+        # Heading text capture for validation constraints
+        markitect schema-generate --capture-heading-text document.md
+        markitect schema-generate --mode outline --capture-heading-text --depth 2 document.md
+
     Modes:
         Default: Standard schema generation with structural analysis
         Outline: Structure-focused schema with heading text capture and metaschema extensions
+
+    Heading Text Capture:
+        When --capture-heading-text is enabled, the schema will include exact heading text
+        as enum constraints, enabling validation to enforce specific heading text requirements.
     """
     try:
         # Handle parameter conflicts and defaults
@@ -1507,7 +1516,8 @@ def generate_schema(config, file_path, max_depth, output, outfile, output_format
             file_path,
             max_depth=final_depth,
             mode=mode,
-            outline_depth=depth if mode == 'outline' else None
+            outline_depth=depth if mode == 'outline' else None,
+            capture_heading_text=capture_heading_text
         )
 
         # Format output