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/02-version-control-as-a-safety-net/README.md

@@ -204,7 +204,7 @@ and your AI assistant.
 
    This is the habit that replaces "paste it back and hope." You're reading exactly what changed,
    nothing more, nothing less. Confirm it does what you asked and didn't touch anything it shouldn't.
-   Run it (`python cli.py count`), then commit:
+   Run it (`python3 cli.py count`), then commit:
 
    ```bash
    git add .
@@ -223,7 +223,7 @@ and your AI assistant.
    git status                 # shows tasks.py as modified
    git restore tasks.py       # discard the change; back to your last commit, byte for byte
    git diff                   # empty: nothing changed. you're clean.
-   python cli.py list         # works again
+   python3 cli.py list         # works again
    ```
 
    You just recovered from a bad AI change in one command, with zero retyping and zero guesswork.
@@ -258,7 +258,7 @@ and your AI assistant.
 
 9. Close the loop and leave the repo clean. The cold session just told you what's in progress and
    what to do next: finish the `delete <index>` command. Do that with the AI (paste in `cli.py` the
-   same way as Part B), run it to confirm it works (`python cli.py delete 1`), then commit:
+   same way as Part B), run it to confirm it works (`python3 cli.py delete 1`), then commit:
 
    ```bash
    git add .