feat: Complete Issue #50 - Define metaschema for JSON schema structure

Implement comprehensive MarkiTect metaschema that extends standard JSON Schema
with MarkiTect-specific features for document analysis and generation.

🎯 TDD8 Implementation Complete:
- ISSUE: Analyzed existing schema system and requirements
- TEST: 15 comprehensive tests covering all features
- RED: Verified tests fail before implementation
- GREEN: Implemented metaschema JSON and validation logic
- REFACTOR: Clean, extensible validator architecture
- DOCUMENT: Updated CLI help and comprehensive documentation
- REFINE: 100% test success rate and CLI integration
- PUBLISH: Ready for production use

✅ Key Features Implemented:
- Heading text capture support (x-markitect-heading-text)
- Content field instructions (x-markitect-content-instructions)
- Outline structure representation (x-markitect-outline-mode/depth)
- Backward compatibility with existing schemas
- Validation rules for all new features
- CLI integration in schema-ingest command

📁 Files Added:
- markitect/metaschema.py - Validation logic and MetaschemaValidator
- markitect/schemas/markitect-metaschema.json - Metaschema definition
- Enhanced markitect/cli.py - Automatic metaschema validation

🧪 Testing:
- 15 comprehensive tests (100% passing)
- RED-GREEN-REFACTOR cycle validated
- CLI integration tested and working
- Backward compatibility verified

📋 Acceptance Criteria Met:
✅ Schema metaschema supports heading text capture
✅ Schema metaschema supports content field instructions
✅ Schema metaschema supports outline structure representation
✅ Schema metaschema is backward compatible with existing schemas
✅ Schema metaschema includes validation rules for new features
✅ Documentation explains the metaschema structure and usage

🔗 Foundation for Future Issues:
- Issue #51: Outline mode schema generation
- Issue #52: Heading text capture in schemas
- Issue #54: Content instruction capabilities
- Issue #55: Schema-based draft generation

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

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

markitect/cli.py

@@ -1653,14 +1653,16 @@ def schema_ingest(config, schema_file, name):
     """
     Read and store a JSON schema file in the database.
 
-    Implements Issue #3 functionality to ingest external schema files
-    and store them for later use with validation and other operations.
+    Validates schemas against the MarkiTect metaschema to ensure compatibility
+    with MarkiTect features like heading text capture and content instructions.
+    Implements Issue #3 and Issue #50 functionality.
 
     SCHEMA_FILE: Path to the JSON schema file to store
 
     Examples:
         markitect schema-ingest my_schema.json
         markitect schema-ingest external_schema.json --name custom-name
+        markitect schema-ingest markitect_schema.json -v    # Show metaschema validation
     """
     try:
         # Determine schema name
@@ -1677,6 +1679,25 @@ def schema_ingest(config, schema_file, name):
             click.echo(f"Error: Invalid JSON in schema file - {e}", err=True)
             sys.exit(1)
 
+        # Validate against MarkiTect metaschema
+        from .metaschema import MetaschemaValidator
+        try:
+            metaschema_validator = MetaschemaValidator()
+            validation_result = metaschema_validator.validate_schema_with_errors(schema_data)
+
+            if not validation_result.is_valid:
+                click.echo("⚠️  Schema validation warnings against MarkiTect metaschema:", err=True)
+                for error in validation_result.errors:
+                    click.echo(f"   - {error.message}", err=True)
+                click.echo("   Schema will be stored but may not be fully compatible with MarkiTect features.", err=True)
+            else:
+                if config.get('verbose'):
+                    click.echo("✅ Schema validates successfully against MarkiTect metaschema")
+
+        except Exception as e:
+            if config.get('verbose'):
+                click.echo(f"⚠️  Could not validate against metaschema: {e}", err=True)
+
         # Initialize database and store schema
         from .database import DatabaseManager
         db_path = config.get('database', 'markitect.db')