feat: implement Phase 1 - Enhanced Schema Format with Classifications

Complete Phase 1 of Schema Evolution Workplan implementing flexible content
control and section classification system.

## New Features

### 1. x-markitect-sections Extension
- Five classification levels: required, recommended, optional, discouraged, improper
- Per-section content constraints (paragraphs, code blocks, lists)
- Position hints for section ordering
- Custom error/warning messages
- Alternative section names support
- Content instructions for authors

### 2. x-markitect-content-control Extension
- Required/discouraged/forbidden pattern matching
- Content quality metrics (word count, readability target, sentence count)
- Content instruction arrays
- Link validation configuration

### 3. Metaschema Validation
- Updated markitect-metaschema.json with complete validation rules
- Enhanced metaschema.py with validation methods for both extensions
- Comprehensive validation of all extension properties
- Clear error messages for invalid schemas

### 4. Documentation & Examples
- Complete specification in docs/specifications/schema-extensions-spec.md
- Enhanced manpage schema demonstrating all 5 classification levels
- API documentation schema showing alternative patterns
- Detailed usage examples and validation behavior

## Implementation Details

**Files Modified:**
- markitect/schemas/markitect-metaschema.json: Added extension definitions
- markitect/metaschema.py: Added _validate_sections() and _validate_content_control()

**Files Created:**
- docs/specifications/schema-extensions-spec.md: Complete specification (v1.0)
- examples/manpages/enhanced-manpage-schema.json: Demonstrates all classifications
- examples/manpages/api-documentation-schema.json: Shows API doc patterns

## Validation Behavior

**Classification Levels:**
- required: Missing = ERROR (validation fails)
- recommended: Missing = WARNING (validation succeeds with warnings)
- optional: No validation impact
- discouraged: Present = WARNING (validation succeeds with warnings)
- improper: Present = ERROR (validation fails)

## Next Steps

Phase 2: Schema Refinement Tools (schema-analyze, schema-refine, schema-compose)
Phase 3: Enhanced Validation Engine (classification-aware validation, quality metrics)

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

markitect/schemas/markitect-metaschema.json

@@ -40,6 +40,163 @@
           "type": "string",
           "enum": ["outline", "full"],
           "description": "Mode used to generate this schema"
+        },
+        "x-markitect-sections": {
+          "type": "object",
+          "description": "Section classification and content control for document sections",
+          "patternProperties": {
+            "^[A-Z][A-Z0-9_ ]*$": {
+              "type": "object",
+              "description": "Section definition with classification and constraints",
+              "properties": {
+                "classification": {
+                  "type": "string",
+                  "enum": ["required", "recommended", "optional", "discouraged", "improper"],
+                  "description": "Classification level determining validation behavior"
+                },
+                "heading_level": {
+                  "type": "integer",
+                  "minimum": 1,
+                  "maximum": 6,
+                  "description": "Expected heading level (H1-H6) for this section"
+                },
+                "position": {
+                  "type": "string",
+                  "enum": ["after_title", "before_section_name", "after_section_name", "anywhere"],
+                  "description": "Where this section should appear in the document"
+                },
+                "content_instruction": {
+                  "type": "string",
+                  "description": "Human-readable instruction for section content"
+                },
+                "min_paragraphs": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Minimum number of paragraphs in this section"
+                },
+                "max_paragraphs": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Maximum number of paragraphs in this section"
+                },
+                "min_code_blocks": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Minimum number of code blocks in this section"
+                },
+                "max_code_blocks": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Maximum number of code blocks in this section"
+                },
+                "min_lists": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Minimum number of lists in this section"
+                },
+                "max_lists": {
+                  "type": "integer",
+                  "minimum": 0,
+                  "description": "Maximum number of lists in this section"
+                },
+                "warning_if_missing": {
+                  "type": "string",
+                  "description": "Custom warning message for missing recommended sections"
+                },
+                "error_message": {
+                  "type": "string",
+                  "description": "Custom error message for required/improper section violations"
+                },
+                "alternatives": {
+                  "type": "array",
+                  "items": {"type": "string"},
+                  "description": "Alternative section names that satisfy the requirement"
+                }
+              },
+              "required": ["classification"]
+            }
+          }
+        },
+        "x-markitect-content-control": {
+          "type": "object",
+          "description": "Content validation rules including patterns and quality metrics",
+          "patternProperties": {
+            "^[a-z][a-z0-9_]*$": {
+              "type": "object",
+              "description": "Content control rules for a specific section",
+              "properties": {
+                "required_patterns": {
+                  "type": "array",
+                  "items": {"type": "string"},
+                  "description": "Regex patterns that must appear in section content"
+                },
+                "discouraged_patterns": {
+                  "type": "array",
+                  "items": {"type": "string"},
+                  "description": "Regex patterns that should not appear in content (warning)"
+                },
+                "forbidden_patterns": {
+                  "type": "array",
+                  "items": {"type": "string"},
+                  "description": "Regex patterns that must not appear in content (error)"
+                },
+                "content_quality": {
+                  "type": "object",
+                  "description": "Quality metrics for section content",
+                  "properties": {
+                    "min_words": {
+                      "type": "integer",
+                      "minimum": 0,
+                      "description": "Minimum word count"
+                    },
+                    "max_words": {
+                      "type": "integer",
+                      "minimum": 0,
+                      "description": "Maximum word count"
+                    },
+                    "readability_target": {
+                      "type": "string",
+                      "enum": ["simple", "general", "technical", "advanced"],
+                      "description": "Target readability level"
+                    },
+                    "min_sentences": {
+                      "type": "integer",
+                      "minimum": 0,
+                      "description": "Minimum sentence count"
+                    },
+                    "max_sentences": {
+                      "type": "integer",
+                      "minimum": 0,
+                      "description": "Maximum sentence count"
+                    }
+                  }
+                },
+                "content_instructions": {
+                  "type": "array",
+                  "items": {"type": "string"},
+                  "description": "Array of human-readable content creation instructions"
+                },
+                "link_validation": {
+                  "type": "object",
+                  "description": "Link checking configuration",
+                  "properties": {
+                    "check_internal": {
+                      "type": "boolean",
+                      "description": "Validate internal document links"
+                    },
+                    "check_external": {
+                      "type": "boolean",
+                      "description": "Validate external URLs"
+                    },
+                    "allow_fragments": {
+                      "type": "boolean",
+                      "description": "Allow fragment-only links like #section"
+                    }
+                  }
+                }
+              }
+            }
+          }
         }
       },
       "patternProperties": {