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>

stack.env.example

@@ -43,6 +43,9 @@ QUADLET_PROSODY_CERTS_DIR=./.podman/gia_prosody_certs
 QUADLET_PROSODY_DATA_DIR=./.podman/gia_prosody_data
 QUADLET_PROSODY_LOGS_DIR=./.podman/gia_prosody_logs
 
+# Signal CLI account number (E.164 format, e.g. +447700900000). Required for Signal integration.
+SIGNAL_NUMBER=
+
 # Memory/wiki search backend foundation
 MEMORY_SEARCH_BACKEND=django
 MANTICORE_HTTP_URL=http://localhost:9308