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_adversarial_v3.py

@@ -5,11 +5,7 @@ comparisons, 'difference between X and Y' phrasing, trailing context
 leaking into entities, degenerate inputs, and false-positive resistance.
 """
 
-import sys
 import unittest
-from pathlib import Path
-
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "skills" / "last30days" / "scripts"))
 
 from lib import planner
 
@@ -193,6 +189,5 @@ class TestNoiseWordEntities(unittest.TestCase):
         entities = planner._comparison_entities("Swift vs Rust vs Go")
         self.assertTrue(any("Go" in e for e in entities))
 
-
 if __name__ == "__main__":
     unittest.main()