feat(schema): add semantic schema generation as default mode

schema-generate now builds content-aware schemas from the document's
section hierarchy instead of counting markdown syntax elements. Detects
key-value tables, data tables, link lists, and mixed content patterns
to produce schemas that reflect the actual document outline.

Old behavior preserved via --mode syntactic. Validator and visualization
tools pinned to syntactic mode for compatibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_issue_51_outline_mode.py

@@ -290,7 +290,7 @@ This is a test document.
                 output_file.unlink()
 
     def test_cli_maintains_backward_compatibility_with_max_depth(self):
-        """Test that existing --max-depth option still works with default mode."""
+        """Test that existing --max-depth option still works with default (semantic) mode."""
         # Arrange
         markdown_content = """# Test Document
 
@@ -317,9 +317,9 @@ Some details here.
             assert result.exit_code == 0, f"CLI should maintain backward compatibility with --max-depth, got: {result.output}"
             schema = json.loads(result.output)
 
-            # Should use old title format for backward compatibility
-            expected_title = f"Schema for {temp_file.name}"
-            assert schema["title"] == expected_title, f"Default mode should use 'for' in title"
+            # Default mode is now semantic, which uses 'from' in title
+            expected_title = f"Schema from {temp_file.name}"
+            assert schema["title"] == expected_title, f"Default (semantic) mode should use 'from' in title"
 
         finally:
             temp_file.unlink()