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_grounding_v3.py

@@ -1,10 +1,6 @@
-import sys
 import unittest
-from pathlib import Path
 from unittest.mock import patch
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "skills" / "last30days" / "scripts"))
-
 from lib import grounding
 
 
@@ -338,6 +334,5 @@ class RedditEnrichItemsTests(unittest.TestCase):
             msg=f"Expected a rate-limit stderr message, got: {captured_stderr!r}",
         )
 
-
 if __name__ == "__main__":
     unittest.main()