feat: add semantic document validator for x-markitect extensions

Implements semantic validation to complement existing structural validation:

Phase 1 & 2 Complete:
- SemanticValidator: Main validator orchestrating sub-validators
- SectionValidator: Enforces section classifications (required, recommended,
  optional, discouraged, improper) from x-markitect-sections
- ContentValidator: Validates content patterns, forbidden patterns, and
  quality metrics (word counts, sentence counts) from x-markitect-content-control

Features:
- Pattern matching with regex for required/forbidden/discouraged patterns
- Word count and sentence count validation
- Detailed error reporting with severity levels (ERROR, WARNING)
- Support for section alternatives (e.g., FLAGS vs OPTIONS)
- Comprehensive test coverage (16 tests, 100% passing)

Architecture:
- Complements existing SchemaValidator (structural AST validation)
- Clean separation: validators/ package for modular validators
- Semantic validation focuses on x-markitect-* extensions
- LinkValidator planned for Phase 3 (optional --check-links)

Next: Phase 4 - CLI integration to enhance 'markitect validate' command

Workplan: roadmap/20260106-semantic-document-validation/WORKPLAN.md

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

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

markitect/validators/__init__.py

@@ -0,0 +1,50 @@
+"""
+Validators package for semantic document validation.
+
+This package contains validators that check markdown documents against
+x-markitect schema extensions (sections, content-control, link validation).
+
+Validators:
+    - SectionValidator: Validates section presence based on classifications
+    - ContentValidator: Validates content patterns and quality metrics
+    - LinkValidator: Validates internal and external links
+"""
+
+from markitect.validators.section_validator import (
+    SectionValidator,
+    SectionValidationResult,
+    SectionIssue,
+    SectionMissing,
+    SectionImproper,
+    SectionDiscouraged,
+)
+
+from markitect.validators.content_validator import (
+    ContentValidator,
+    ContentValidationResult,
+    ContentIssue,
+    PatternMissing,
+    ForbiddenPattern,
+    DiscouragedPattern,
+    ContentTooShort,
+    ContentTooLong,
+)
+
+__all__ = [
+    # Section validator
+    'SectionValidator',
+    'SectionValidationResult',
+    'SectionIssue',
+    'SectionMissing',
+    'SectionImproper',
+    'SectionDiscouraged',
+    # Content validator
+    'ContentValidator',
+    'ContentValidationResult',
+    'ContentIssue',
+    'PatternMissing',
+    'ForbiddenPattern',
+    'DiscouragedPattern',
+    'ContentTooShort',
+    'ContentTooLong',
+]