Remove real contact numbers from tests and update tooling

- Replace real phone numbers in tests with Ofcom-reserved fictitious
  numbers (447700900xxx range) throughout test suite
- Add SIGNAL_NUMBER to stack.env.example documenting required env var
- Update pre-commit hooks to latest versions (black 26.3.0, isort 8.0.1,
  flake8 7.3.0, djhtml 3.0.10, ripsecrets v0.1.11)
- Add CLAUDE.md with rule prohibiting real contact identifiers in code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/tests/test_signal_unlink_fallback.py

@@ -40,4 +40,3 @@ class SignalUnlinkFallbackTests(TestCase):
         self.assertTrue(result)
         self.assertEqual(2, mock_delete.call_count)
         mock_wipe.assert_called_once()
-