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>

.pre-commit-config.yaml

@@ -1,22 +1,22 @@
 repos:
   - repo: https://github.com/psf/black
-    rev: 23.1.0
+    rev: 26.3.0
     hooks:
     - id: black
       exclude: ^core/migrations
   - repo: https://github.com/PyCQA/isort
-    rev: 5.11.5
+    rev: 8.0.1
     hooks:
       - id: isort
         args: ["--profile", "black"]
   - repo: https://github.com/PyCQA/flake8
-    rev: 6.0.0
+    rev: 7.3.0
     hooks:
       - id: flake8
         args: [--max-line-length=88]
         exclude: ^core/migrations
   - repo: https://github.com/rtts/djhtml
-    rev: v2.0.0
+    rev: 3.0.10
     hooks:
       - id: djhtml
         args: [-t 2]
@@ -25,6 +25,6 @@ repos:
       - id: djjs
         exclude: ^core/static/js # slow
   - repo: https://github.com/sirwart/ripsecrets.git
-    rev: v0.1.5
+    rev: v0.1.11
     hooks:
       - id: ripsecrets