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_5_schema_generation.py

@@ -50,8 +50,8 @@ Some text here.
             temp_file = Path(f.name)
 
         try:
-            # Act - Generate schema with unlimited depth
-            result = self.schema_generator.generate_schema_from_file(temp_file)
+            # Act - Generate schema in syntactic mode (element counting)
+            result = self.schema_generator.generate_schema_from_file(temp_file, mode='syntactic')
 
             # Assert - Schema should be valid JSON and contain expected structure
             assert isinstance(result, dict)
@@ -105,8 +105,8 @@ Very deep content.
             temp_file = Path(f.name)
 
         try:
-            # Act - Generate schema with depth limit of 2
-            result = self.schema_generator.generate_schema_from_file(temp_file, max_depth=2)
+            # Act - Generate schema in syntactic mode with depth limit of 2
+            result = self.schema_generator.generate_schema_from_file(temp_file, max_depth=2, mode='syntactic')
 
             # Assert - Only levels 1 and 2 should be included
             properties = result.get("properties", {})
@@ -173,8 +173,8 @@ Some implementation notes here.
             temp_file = Path(f.name)
 
         try:
-            # Act - Generate schema
-            result = self.schema_generator.generate_schema_from_file(temp_file)
+            # Act - Generate schema in syntactic mode
+            result = self.schema_generator.generate_schema_from_file(temp_file, mode='syntactic')
 
             # Assert - Schema should capture complex structures
             properties = result.get("properties", {})