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/16-containers-and-reproducible-environments/README.md

@@ -221,12 +221,13 @@ containerize and run the app you already have.
    ```
 
    That's a clean Python with none of your code. Now confirm CI-grade reproducibility — run the
-   Module 14 test suite in a clean, throwaway container that mounts your code but installs its tools
-   fresh (no test tools baked into your app image — that keeps it lean; see *Where it breaks*):
+   Module 14 test suite in a clean, throwaway container that mounts your code and runs it with the
+   standard-library `unittest` runner: nothing to install, and no test tooling baked into your app
+   image (that keeps it lean; see *Where it breaks*):
 
    ```bash
    docker run --rm -v "$PWD":/app -w /app python:3.12-slim \
-     sh -c "pip install pytest -q && pytest -q"
+     python -m unittest
    ```
 
    This is, in miniature, exactly what containerized CI does. If it passes here, it passes the same