feat(infospace): add entity metadata parser (S1.1)

Extract section-tree algorithm from SchemaGenerator into standalone
core/section_tree.py and build markitect/infospace/ package with
EntityMeta dataclass and parse_entity_file/parse_entity_directory.
Foundation for schema compliance, coverage, and granularity metrics.

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

markitect/core/__init__.py

@@ -9,6 +9,7 @@ This package contains the fundamental building blocks:
 """
 
 from .parser import parse_markdown_to_ast
+from .section_tree import build_section_tree, extract_section_text
 from .serializer import ASTSerializer
 from .document_manager import DocumentManager, CleanDocumentManager
 from .workspace import (
@@ -29,6 +30,9 @@ from .workspace import (
 __all__ = [
     # Parser
     "parse_markdown_to_ast",
+    # Section tree
+    "build_section_tree",
+    "extract_section_text",
     # Serializer
     "ASTSerializer",
     # Document Manager