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_category_integration.py

@@ -13,17 +13,12 @@ Fixture reference: `tests/fixtures/prompting-gpt-image-2-resolved-block.md`.
 """
 
 import io
-import sys
 import unittest
 from contextlib import redirect_stderr
-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 resolve
 
-
 OPENAI_BRAND_SUBREDDIT_RESULTS = [
     {
         "title": "r/OpenAI community hub",
@@ -140,6 +135,5 @@ class PromptingGptImage2RegressionGuard(unittest.TestCase):
         self.assertIsNone(result["category"])
         self.assertNotIn("Matched category=", buf.getvalue())
 
-
 if __name__ == "__main__":
     unittest.main()