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_briefing_v3.py

@@ -1,11 +1,8 @@
-import sys
 import tempfile
 import unittest
 from pathlib import Path
 from unittest import mock
 
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "skills" / "last30days" / "scripts"))
-
 import briefing
 import store
 
@@ -52,6 +49,5 @@ class BriefingV3Tests(unittest.TestCase):
             finally:
                 briefing.BRIEFS_DIR = old_briefs_dir
 
-
 if __name__ == "__main__":
     unittest.main()