feat(metadata): NIL_METADATA sentinel + migrate _build_files_section_from_items

src/aggregate.py

@@ -47,6 +47,9 @@ from src.type_aliases import (
 )
 
 
+NIL_METADATA: Metadata = {}
+
+
 def find_next_increment(output_dir: Path, namespace: str) -> int:
  pattern = re.compile(rf"^{re.escape(namespace)}_(\d+)\.md$")
  max_num = 0
@@ -303,13 +306,15 @@ def _build_files_section_from_items(file_items: list[Metadata]) -> str:
   [C: tests/test_aggregate_flags.py:test_auto_aggregate_skip, tests/test_context_composition_phase6.py:test_files_section_rendering, tests/test_tiered_context.py:test_build_files_section_with_dicts, tests/test_ui_summary_only_removal.py:test_aggregate_from_items_respects_auto_aggregate]
  """
  sections = []
+ file_items = file_items or []
  for item in file_items:
+  item = item or NIL_METADATA
   if not item.get("auto_aggregate", True): continue
   path      = item.get("path")
   entry     = item.get("entry", "unknown")
   content   = item.get("content", "")
   view_mode = item.get("view_mode", "full")
-  if path is None:
+  if not path:
    if view_mode == "summary":
     sections.append(f"### `{entry}`\n\n{content}")
    else: