Use python3 as the canonical command name course-wide (#104)

Most current systems (default Debian/Ubuntu, recent macOS) install Python
only as `python3`, with no bare `python` on PATH, so learners who copied
`python cli.py ...` into their host shell hit "command not found".

Convert host-shell `python <cmd>` -> `python3 <cmd>` across module/lab
READMEs, lab `.py` docstrings & usage strings, blog posts, lab prompt and
instruction files, the M04 verify.sh message, and the M10/M24 lab patches.
Module 01's convention note (and its blog/02 mirror) is rewritten so
`python3` is canonical and `python` is the documented fallback.

Stop-lines respected: Docker image tags (`python:3.12-slim`), `.venv/.../python`
and `...\.venv\Scripts\python.exe` paths, the M20 `"command": "python"`
teaching example and surrounding venv prose, container-internal invocations
(M16/M18 Dockerfiles, M16 README `docker run` examples), and CI-workflow
`run:` steps fed by `actions/setup-python` / `image: python:3.12` are left
as `python` on purpose.

pip was left out of scope: most occurrences are prose or CI/container-internal,
and `pip3` does not fix the PEP 668 externally-managed-environment refusal that
the course already addresses with venvs. The M01 note is worded to stay
consistent with bare `pip` (use whichever pip pairs with your Python).

Build (tools/build_wiki.py) and tools/check.sh both pass.

Closes #104

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GAEzanEoGJT5o1VizQar47

modules/11-collaboration-humans-and-agents/README.md

@@ -355,11 +355,11 @@ the server say *no* is the point: "never commit to `main`" is now a rule, not a
    the CLI), and it does what you asked. Run it:
 
    ```bash
-   python cli.py add "keeper" ; python cli.py add "trash"
-   python cli.py list                   # note the index shown next to "trash"
-   python cli.py done <trash-index>     # use the index "list" just printed, NOT a fixed 1
-   python cli.py clear-done             # expect it to remove the completed one
-   python cli.py list                   # "keeper" remains, "trash" is gone
+   python3 cli.py add "keeper" ; python3 cli.py add "trash"
+   python3 cli.py list                   # note the index shown next to "trash"
+   python3 cli.py done <trash-index>     # use the index "list" just printed, NOT a fixed 1
+   python3 cli.py clear-done             # expect it to remove the completed one
+   python3 cli.py list                   # "keeper" remains, "trash" is gone
    ```
 
    Read the index off `list` rather than assuming it: `done` is positional, and your `tasks-app` has