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>

tools/visualize_schema.py

@@ -20,7 +20,7 @@ def visualize_schema_structure(file_path, max_depth=None, ascii_only=False):
     """Create a beautiful tree visualization of the document structure."""
 
     generator = SchemaGenerator()
-    schema = generator.generate_schema_from_file(Path(file_path), max_depth=max_depth)
+    schema = generator.generate_schema_from_file(Path(file_path), max_depth=max_depth, mode='syntactic')
 
     # Define icons based on ASCII mode
     if ascii_only: