feat(nixos): migrate system configuration

configs/nixos/modules/agents/claude-code/skills/test-driven-development/refactoring.md

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+# Refactor Candidates
+
+After TDD cycle, look for:
+
+- **Duplication** → Extract function/class
+- **Long methods** → Break into private helpers (keep tests on public interface)
+- **Shallow modules** → Combine or deepen — see DEEPENING.md in `/codebase-design` for dependency categories and seam discipline
+- **Feature envy** → Move logic to where data lives
+- **Primitive obsession** → Introduce value objects
+- **Existing code** the new code reveals as problematic