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/validators/__init__.py

@@ -30,6 +30,16 @@ from markitect.validators.content_validator import (
     ContentTooLong,
 )
 
+from markitect.validators.link_validator import (
+    LinkValidator,
+    LinkValidationResult,
+    LinkIssue,
+    BrokenInternalLink,
+    BrokenExternalLink,
+    FragmentNotAllowed,
+    InvalidEmail,
+)
+
 __all__ = [
     # Section validator
     'SectionValidator',
@@ -47,4 +57,12 @@ __all__ = [
     'DiscouragedPattern',
     'ContentTooShort',
     'ContentTooLong',
+    # Link validator
+    'LinkValidator',
+    'LinkValidationResult',
+    'LinkIssue',
+    'BrokenInternalLink',
+    'BrokenExternalLink',
+    'FragmentNotAllowed',
+    'InvalidEmail',
 ]