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_cli_v3.py

@@ -1,4 +1,3 @@
-# ruff: noqa: E402
 import json
 import io
 import shutil
@@ -11,13 +10,11 @@ from contextlib import redirect_stderr, redirect_stdout
 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
 from lib import schema
 
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
 
 class CliV3Tests(unittest.TestCase):
     def make_report(self) -> schema.Report:
@@ -305,6 +302,5 @@ class CliV3Tests(unittest.TestCase):
         )
         self.assertIn("[GitHub] Canonicalized repos:", stderr.getvalue())
 
-
 if __name__ == "__main__":
     unittest.main()