tests: centralize script path setup in conftest.py

Add a pytest-discovered tests/conftest.py for the last30days scripts path and
remove duplicate per-file sys.path.insert boilerplate from tests.

Normalize affected imports to rely on the shared scripts path and remove the
now-unneeded E402 suppressions.

tests/test_plugin_contract.py

@@ -1,16 +1,13 @@
 import json
-import sys
 import tomllib
 import unittest
 from pathlib import Path
 
+from lib.skill_meta import read_skill_version
 
 ROOT = Path(__file__).resolve().parents[1]
 SKILL_ROOT = ROOT / "skills" / "last30days"
 
-sys.path.insert(0, str(SKILL_ROOT / "scripts"))
-from lib.skill_meta import read_skill_version  # noqa: E402
-
 
 def _json(path: Path) -> dict:
     return json.loads(path.read_text(encoding="utf-8"))
@@ -60,6 +57,5 @@ class TestPluginContract(unittest.TestCase):
 
         self.assertEqual([], offenders)
 
-
 if __name__ == "__main__":
     unittest.main()