fix: CI failures — update workflow for uv migration, fix lint and format

- Switch CI install step from `pip install -r requirements.txt` to
  `pip install -e ".[dev]"` since requirements.txt was removed
- Add noqa: E402 to intentionally-late imports in conftest.py
  (HOME must be isolated before mempalace imports)
- Remove unused KnowledgeGraph import in test_knowledge_graph.py
- Apply ruff formatting to test files

tests/test_knowledge_graph.py

@@ -5,8 +5,6 @@ Covers: entity CRUD, triple CRUD, temporal queries, invalidation,
 timeline, stats, and edge cases (duplicate triples, ID collisions).
 """
 
-from mempalace.knowledge_graph import KnowledgeGraph
-
 
 class TestEntityOperations:
     def test_add_entity(self, kg):