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_plan_threading.py

@@ -1,20 +1,15 @@
-# ruff: noqa: E402
 """Tests for --competitors-plan JSON parsing and per-entity kwargs threading."""
 
 from __future__ import annotations
 
 import io
 import json
-import sys
 import tempfile
 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"))
-
 import last30days as cli
 
 
@@ -175,6 +170,5 @@ class SubrunKwargsForTests(unittest.TestCase):
         kwargs = cli.subrun_kwargs_for("X", {}, resolved=resolved)
         self.assertEqual(kwargs["_context"], "Resolved context")
 
-
 if __name__ == "__main__":
     unittest.main()