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_cluster_v3.py

@@ -1,8 +1,4 @@
-import sys
 import unittest
-from pathlib import Path
-
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "skills" / "last30days" / "scripts"))
 
 from lib import cluster, schema
 
@@ -196,6 +192,5 @@ class TestClusterUncertainty(unittest.TestCase):
         result = cluster._cluster_uncertainty(candidates)
         self.assertEqual("thin-evidence", result)
 
-
 if __name__ == "__main__":
     unittest.main()