chore: apply ruff auto-fixes and remove dead AST scripts

tests/test_tiered_context.py

@@ -1,4 +1,3 @@
-import pytest
 from typing import Any
 from pathlib import Path
 from aggregate import build_tier1_context, build_tier2_context, build_tier3_context
@@ -24,8 +23,6 @@ def test_build_tier2_context_exists() -> None:
 
 def test_build_tier3_context_ast_skeleton(monkeypatch: Any) -> None:
  from unittest.mock import MagicMock
- import aggregate
- import file_cache
  # Mock ASTParser
  mock_parser_instance = MagicMock()
  mock_parser_instance.get_skeleton.return_value = "def other():\n    ..."