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/06-commit-the-ai-config.md

@@ -10,7 +10,7 @@ Tags:               AI, developer workflow, version control, configuration, AGEN
 
 # Commit the AI's Config, Not Just the Code
 
-I used to start every AI coding session the same way: by giving the same little speech. "We use four-space indent. Run the tests with `python -m unittest` before you tell me it works. The logic goes in `tasks.py`, not crammed into the CLI file. And whatever you do, don't hand-edit `tasks.json`; it's generated."
+I used to start every AI coding session the same way: by giving the same little speech. "We use four-space indent. Run the tests with `python3 -m unittest` before you tell me it works. The logic goes in `tasks.py`, not crammed into the CLI file. And whatever you do, don't hand-edit `tasks.json`; it's generated."
 
 The AI would nod (figuratively), do exactly that, and we'd have a great session. Then I'd close the tab. The next morning I'd open a fresh one, and the AI had forgotten every word of it. So I'd give the speech again. And again. I was a broken record reading my own project back to a goldfish.
 
@@ -27,7 +27,7 @@ Different vendors look for different filenames, and honestly, the names keep cha
 So what goes in it? Not a prompt, and not your README. This is a briefing for an agent that's about to edit your code. Keep it to things that actually change the AI's behavior:
 
 - **Project conventions**: the layout and patterns this codebase actually uses. *"Core logic lives in `tasks.py`; the CLI front end is `cli.py`; state persists to `tasks.json`."*
-- **Build and test commands**: the exact, copy-pasteable commands. *"Run tests with `python -m unittest`. Don't claim a change works until they pass."* That one line stops the AI from inventing a test runner you don't use.
+- **Build and test commands**: the exact, copy-pasteable commands. *"Run tests with `python3 -m unittest`. Don't claim a change works until they pass."* That one line stops the AI from inventing a test runner you don't use.
 - **Coding standards**: *"Standard library only, no third-party packages. Type-hint public functions."*
 - **The don't-touch list**: generated files, vendored code, secrets. *"Never edit `tasks.json` by hand; it's generated."*
 - **House style**: the taste calls that otherwise come back wrong every time. *"Keep functions small. Don't reformat files you aren't changing."*