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

blog/14-unit3-automate-checking-shipping.md

@@ -33,7 +33,7 @@ That "one done" case is the one where a correct implementation and a buggy one g
 
 A test file sitting in your repo is useful right up until you forget to run it, which, like every manual check, you eventually will. Continuous Integration removes the "eventually." It's a grand name for a mundane core: **the same checks you'd run by hand (lint, build, test) bound to a trigger, on a clean machine you don't control, on every single push.**
 
-The magic is entirely in *automatically*. You don't run CI; pushing runs it. It can't be skipped by forgetting, it doesn't get tired on the fortieth push of the day, and its whole enforcement mechanism is the humble exit code: `python -m unittest` returns non-zero when a test fails, and one non-zero turns the run red. The actual config is shorter than this paragraph:
+The magic is entirely in *automatically*. You don't run CI; pushing runs it. It can't be skipped by forgetting, it doesn't get tired on the fortieth push of the day, and its whole enforcement mechanism is the humble exit code: `python3 -m unittest` returns non-zero when a test fails, and one non-zero turns the run red. The actual config is shorter than this paragraph:
 
 ```yaml
 name: CI