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_competitors.py

@@ -1,18 +1,12 @@
-# ruff: noqa: E402
 """Tests for scripts/lib/competitors.discover_competitors."""
 
 from __future__ import annotations
 
 import io
-import sys
 import unittest
 from contextlib import redirect_stderr
-from pathlib import Path
 from unittest import mock
 
-REPO_ROOT = Path(__file__).resolve().parents[1]
-sys.path.insert(0, str(REPO_ROOT / "skills" / "last30days" / "scripts"))
-
 from lib import competitors
 
 
@@ -23,7 +17,6 @@ def _serp(items: list[tuple[str, str]]) -> list[dict]:
         for title, snippet in items
     ]
 
-
 OPENAI_SERP = _serp(
     [
         ("OpenAI vs Anthropic vs xAI: which is better?", "xAI and Anthropic now compete directly with OpenAI."),
@@ -147,6 +140,5 @@ class CompetitorDiscoveryTests(unittest.TestCase):
         results = self._run(OPENAI_SERP, "OpenAI", count=0)
         self.assertEqual(results, [])
 
-
 if __name__ == "__main__":
     unittest.main()