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/20-mcp-servers-giving-the-ai-hands/lab/mcp-config-example.json

@@ -1,8 +1,8 @@
 {
-  "_comment": "Common shape of an MCP server entry for a local (stdio) server. Many agentic tools accept this 'mcpServers' map; yours may use a different key or location (check its docs). Replace the path with the ABSOLUTE path to tasks_mcp_server.py in your tasks-app. Use 'python3' instead of 'python' if that's what your system calls it, or the full path to a virtualenv's python.",
+  "_comment": "Common shape of an MCP server entry for a local (stdio) server. Many agentic tools accept this 'mcpServers' map; yours may use a different key or location (check its docs). IMPORTANT: 'command' must be the ABSOLUTE path to the python interpreter that has the MCP SDK installed (e.g. your venv's python) -- a bare 'python' makes the client launch whatever is on its PATH, which usually does NOT have the SDK, and the server then reports 'not connected'. On Windows the venv python is ...\\.venv\\Scripts\\python.exe. Set 'args' to the ABSOLUTE path to tasks_mcp_server.py in your tasks-app.",
   "mcpServers": {
     "tasks": {
-      "command": "python",
+      "command": "/ABSOLUTE/PATH/TO/workflow-course/tasks-app/.venv/bin/python",
       "args": ["/ABSOLUTE/PATH/TO/workflow-course/tasks-app/tasks_mcp_server.py"]
     }
   }