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_instagram.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.instagram import _parse_items
 
@@ -63,6 +59,5 @@ class TestExpandInstagramQueries(unittest.TestCase):
         queries = expand_instagram_queries("Kanye West", "quick")
         self.assertEqual(len(queries), 1)
 
-
 if __name__ == "__main__":
     unittest.main()