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/05-commit-the-ai-config/lab/instructions-file-starter.md

@@ -26,7 +26,7 @@ minute but real enough to have more than one file. Keep it that way — don't gr
 ## Build and test commands
 
 - Run the app: `python cli.py <command>` (e.g. `python cli.py list`).
-- Run the tests: `pytest`  <!-- EDIT: set this to your real test command, or delete if you have no tests yet -->
+- Run the tests: `python -m unittest`  <!-- EDIT: set this to your real test command, or delete if you have no tests yet -->
 - Do not claim a change works until you have actually run it. If tests exist, they must pass first.
 
 ## Coding standards