fix(testing/ci/tooling): consistent unittest, venv guidance, runnable lab commands

- #9: standardize the test chain on stdlib unittest (nothing-to-install, which
  keeps M13's claims true and its planted bug intact). Aligned M5/M14/M16 prose,
  M14 lab/test_tasks.py, and ci/gitlab starters; ruff stays the only pip install.
- #20: add venv / PEP 668 / which-python guidance to M20 (+ M14/M15 local
  installs); point MCP config at the venv's absolute python.
- #21: replace M21 Part D's empty `git diff HEAD~1` with `git log -p` (no
  .gitignore added — device preserved).
- #22: add a dependency-install step before M23's green baseline on a fresh clone.
- #23: M24 reviewer/triage now tolerate code-fence-wrapped JSON (stdlib only);
  feature.patch trap untouched.
- #28: fix M27 Part D CI snippet path (working-directory) and require the gate to
  target a varying candidate; swapped_model regression kept as the fixture.

Closes #9
Closes #20
Closes #21
Closes #22
Closes #23
Closes #28

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfzV5QvtPDz8LJS3Pu5VLT

modules/14-continuous-integration/lab/ci-starter.yml

@@ -33,14 +33,16 @@ jobs:
         with:
           python-version: "3.12"
 
-      # Step 3: install the tools the checks need — the test runner and the linter from Module 13.
+      # Step 3: install the linter (ruff), the new tool this module adds. The test runner is
+      # Python's standard-library unittest from Module 13 — nothing to install for it.
       - name: Install tools
-        run: pip install pytest ruff
+        run: pip install ruff
 
       # Step 4: lint. Style and obvious-mistake check. Fails the job on any finding (non-zero exit).
       - name: Lint
         run: ruff check .
 
-      # Step 5: test. The Module 13 tests. A single failing assertion fails the whole job.
+      # Step 5: test. The Module 13 tests, run with the stdlib unittest runner. A single failing
+      # assertion fails the whole job.
       - name: Test
-        run: pytest -q
+        run: python -m unittest