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/schemas/markitect-metaschema.json

@@ -0,0 +1,52 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$id": "https://markitect.io/schemas/markitect-metaschema.json",
+  "type": "object",
+  "title": "MarkiTect Extended JSON Schema Metaschema",
+  "description": "Metaschema for MarkiTect JSON schemas that extends standard JSON Schema with MarkiTect-specific features for document structure analysis and generation",
+  "allOf": [
+    {
+      "$ref": "http://json-schema.org/draft-07/schema#"
+    },
+    {
+      "properties": {
+        "x-markitect-heading-text": {
+          "type": "string",
+          "description": "Preserve actual heading text from source document for validation and template generation"
+        },
+        "x-markitect-content-instructions": {
+          "type": "string",
+          "description": "Instructions for content authors about what should go in this section"
+        },
+        "x-markitect-outline-mode": {
+          "type": "boolean",
+          "description": "Indicates if this schema was generated in outline mode, focusing on structural hierarchy"
+        },
+        "x-markitect-outline-depth": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Maximum heading depth captured in outline mode"
+        },
+        "x-markitect-instruction-type": {
+          "type": "string",
+          "enum": ["description", "example", "constraint", "template"],
+          "description": "Type of content instruction provided"
+        },
+        "x-markitect-generated-from": {
+          "type": "string",
+          "description": "Source file or document this schema was generated from"
+        },
+        "x-markitect-generation-mode": {
+          "type": "string",
+          "enum": ["outline", "full"],
+          "description": "Mode used to generate this schema"
+        }
+      },
+      "patternProperties": {
+        "^x-markitect-": {
+          "description": "MarkiTect extension properties"
+        }
+      }
+    }
+  ]
+}