feat: add LinkValidator for semantic link validation (Phase 3)

Implement comprehensive link validation as part of semantic validation:

Core Features:
- Link classification: internal, external, fragment, email
- Internal link validation: fragment anchors and file paths
- External link validation: HTTP/HTTPS with configurable timeout
- Email validation: mailto: link format checking
- Fragment policy enforcement: allow/disallow fragment identifiers

Link Validator:
- markitect/validators/link_validator.py - Full link validation implementation
- Supports x-markitect-content-control.link_validation configuration
- Default: check internal links, skip external (fast)
- Opt-in external checking with --check-links flag

Integration:
- Updated SemanticValidator to include link_result in reports
- CLI already supports --check-links flag (line 1629 in cli.py)
- Link validation runs by default for internal links (fast)
- External link checking requires explicit --check-links flag

Test Coverage:
- Added 9 comprehensive tests for LinkValidator
- Tests cover: classification, broken links, fragments, email, statistics
- All 25 semantic validator tests passing (100%)

Documentation:
- Updated SCHEMA_MANAGEMENT_GUIDE.md with link validation section
- Added examples for broken links and external link checking
- Documented link types, validation rules, and configuration

Statistics Tracking:
- Links checked, internal/external/fragment/email counts
- Detailed error/warning reporting with line numbers
- Integration with existing semantic validation reporting

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

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

markitect/semantic_validator.py

@@ -22,6 +22,10 @@ from markitect.validators.content_validator import (
     ContentValidator,
     ContentValidationResult
 )
+from markitect.validators.link_validator import (
+    LinkValidator,
+    LinkValidationResult
+)
 
 
 @dataclass
@@ -33,7 +37,7 @@ class SemanticValidationReport:
     """
     section_result: SectionValidationResult
     content_result: Optional[ContentValidationResult] = None
-    link_result: Optional[Any] = None  # LinkValidationResult when implemented
+    link_result: Optional[LinkValidationResult] = None
 
     def has_errors(self) -> bool:
         """Check if there are any ERROR-level issues."""
@@ -99,6 +103,17 @@ class SemanticValidationReport:
             else:
                 lines.append("  ✅ All content requirements met")
 
+        # Link validation
+        if self.link_result:
+            lines.append("")
+            lines.append("Link Validation:")
+            if self.link_result.issues:
+                for issue in self.link_result.issues:
+                    status = "❌" if issue.severity == 'ERROR' else "⚠️"
+                    lines.append(f"  {status} {issue.link} - {issue.message}")
+            else:
+                lines.append(f"  ✅ All {self.link_result.links_checked} links valid")
+
         # Summary
         lines.append("")
         lines.append("Summary:")
@@ -112,6 +127,10 @@ class SemanticValidationReport:
             all_errors.extend(self.content_result.get_errors())
             all_warnings.extend(self.content_result.get_warnings())
 
+        if self.link_result:
+            all_errors.extend(self.link_result.get_errors())
+            all_warnings.extend(self.link_result.get_warnings())
+
         lines.append(f"  Errors: {len(all_errors)}")
         lines.append(f"  Warnings: {len(all_warnings)}")
 
@@ -155,9 +174,7 @@ class SemanticValidator:
         # Initialize sub-validators
         self.section_validator = SectionValidator(schema)
         self.content_validator = ContentValidator(schema)
-
-        # TODO: Initialize link validator when implemented
-        # self.link_validator = LinkValidator(schema)
+        self.link_validator = LinkValidator(schema)
 
     def validate(self, document_path: str | Path,
                  check_links: bool = False) -> SemanticValidationReport:
@@ -189,12 +206,12 @@ class SemanticValidator:
         # Run content validation
         content_result = self.content_validator.check(document)
 
-        # TODO: Run link validation when implemented
-        # if check_links:
-        #     link_result = self.link_validator.check(document)
-        # else:
-        #     link_result = None
-        link_result = None
+        # Run link validation (if enabled)
+        if check_links:
+            link_result = self.link_validator.check(document, check_external=True)
+        else:
+            # Still check internal links by default (fast)
+            link_result = self.link_validator.check(document, check_external=False)
 
         return SemanticValidationReport(
             section_result=section_result,