chore: add pre-commit hook with ruff and pytest

- Add git pre-commit hook running ruff check, ruff format, pytest
- Fix import ordering in conftest.py
- Add placeholder test for test infrastructure
- Generate flake.lock

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1766807290,
+        "narHash": "sha256-7sgUQMKMDvpeu9cwE7mWJfP9N3wJduunmJ/s8LOd9nk=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "4eec65df50d796a8fd9c146a09b5007916ce376b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}