ci: run gate_stamp's tests (scripts/tests) in shared-gates CI

They were only reachable through test_changed.sh. Add them to the
shared-gates workflow (with pre-commit, and apt PyYAML for the
/usr/bin/python3 hook-id helper) and to CLAUDE.md's test command.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PAhbNszs5JKdtnD4uBi3xp

.github/workflows/shared-gates-tests.yml

@@ -62,3 +62,12 @@ jobs:
             --cov=coverage_gaps --cov-branch \
             --cov-report=term-missing \
             --cov-fail-under=100
+
+      # gate_stamp.sh is the pre-push gate in every repo. Its tests drive real
+      # pre-commit in throwaway repos, and the hook-id helper runs under
+      # /usr/bin/python3 (the deployment interpreter), hence apt's PyYAML.
+      - name: gate_stamp (step / full --list)
+        run: |
+          sudo apt-get update -q && sudo apt-get install -y python3-yaml
+          python -m pip install --upgrade pre-commit
+          PYTHONPATH=. python -m pytest scripts/tests -q

CLAUDE.md

@@ -10,7 +10,7 @@ overwrite it with `repo_contract/templates/check_repo_contract.shim.sh`.
 ## Commands
 
 - run: n/a: collection of gates and libraries, no single entry point
-- test: `PYTHONPATH=. python -m pytest -q coverage_gaps/tests dep_freshness/tests file_length/tests home_paths/tests md_naming/tests repo_contract/tests`
+- test: `PYTHONPATH=. python -m pytest -q coverage_gaps/tests dep_freshness/tests file_length/tests home_paths/tests md_naming/tests repo_contract/tests scripts/tests`
 - test-changed: `scripts/test_changed.sh`
 - lint: `ruff check coverage_gaps repo_contract`
 - coverage: `PYTHONPATH=. python -m pytest -q repo_contract/tests --cov=repo_contract --cov-branch --cov-report=xml --cov-fail-under=100`