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>

docs/SCHEMA_MANAGEMENT_GUIDE.md

@@ -177,6 +177,9 @@ markitect validate my-document.md --schema manpage-schema-v1.0.md
 # Only structural validation (classic mode)
 markitect validate my-document.md --schema schema.json --no-semantic
 
+# With external link checking (may be slow)
+markitect validate my-document.md --schema manpage-schema-v1.0.md --check-links
+
 # Strict mode (warnings become errors)
 markitect validate my-document.md --schema manpage-schema-v1.0.md --strict
 ```
@@ -202,6 +205,14 @@ markitect validate my-document.md --schema manpage-schema-v1.0.md --strict
 - **Quality Metrics**: Checks word counts, sentence counts
   - `min_words`, `max_words`: Word count requirements (WARNING)
   - `min_sentences`: Minimum sentence count (WARNING)
+- **Link Validation**: Validates internal and external links (optional)
+  - Internal links: Checked by default when semantic validation enabled
+    - Fragment links (#section-name) verified to exist (ERROR if broken)
+    - Relative file paths checked for existence (ERROR if broken)
+  - External links: Opt-in with --check-links flag (may be slow)
+    - HTTP/HTTPS URLs validated with HEAD requests (WARNING if broken)
+  - Email validation: Validates mailto: link format (WARNING if invalid)
+  - Fragment policy: Configurable allow/disallow fragment identifiers
 
 ### Validation Output
 
@@ -222,6 +233,9 @@ Section Validation:
 Content Validation:
   ✅ All content requirements met
 
+Link Validation:
+  ✅ All 12 links valid
+
 Summary:
   Sections checked: 3
   Sections found: 5
@@ -271,6 +285,30 @@ $ markitect validate doc.md --schema manpage-schema-v1.0.md --strict
 Status: FAILED ❌  (warnings treated as errors)
 ```
 
+**Example 4: Broken Internal Link**
+```bash
+$ markitect validate doc.md --schema manpage-schema-v1.0.md
+
+Link Validation:
+  ❌ #nonexistent-section - Internal link target not found: #nonexistent-section
+
+Errors: 1
+Status: FAILED ❌
+```
+
+**Example 5: External Link Validation**
+```bash
+# Enable external link checking (may be slow)
+$ markitect validate doc.md --schema manpage-schema-v1.0.md --check-links
+
+Link Validation:
+  ✅ http://example.com - Valid
+  ⚠️  http://broken-link.invalid - External link unreachable: Name or service not known
+
+Warnings: 1
+Status: PASSED ✅
+```
+
 ## Schema Naming Conventions
 
 All schema filenames must follow this pattern: