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/17-capstone-the-full-loop.md

@@ -22,7 +22,7 @@ If you've been following the series here on the blog, this is the part where the
 
 Here's the trick that makes a capstone honest: pick something *small* enough to finish in one sitting but *real* enough to touch the whole stack. We're adding due dates to the running `tasks-app`:
 
-- A task can carry an optional due date: `python cli.py add "file taxes" --due 2026-09-15`.
+- A task can carry an optional due date: `python3 cli.py add "file taxes" --due 2026-09-15`.
 - A new `overdue` command lists pending tasks whose due date has already passed.
 - The deployed service grows a matching `GET /overdue` endpoint, so the change is visible in the *running container*, not just the CLI.