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/09-issues-and-the-task-layer/lab/example-issues.md

@@ -18,16 +18,16 @@
 
 ## Context / problem
 
-`python cli.py done 99` on a list with 3 tasks raises an uncaught `IndexError` and dumps a Python
-traceback. `python cli.py done abc` raises `ValueError` the same way. The user sees a stack trace
+`python3 cli.py done 99` on a list with 3 tasks raises an uncaught `IndexError` and dumps a Python
+traceback. `python3 cli.py done abc` raises `ValueError` the same way. The user sees a stack trace
 instead of a helpful message, and the process exits as if it crashed.
 
 Reproduce:
 
 ```
-python cli.py add "first"
-python cli.py done 99      # IndexError traceback
-python cli.py done abc     # ValueError traceback
+python3 cli.py add "first"
+python3 cli.py done 99      # IndexError traceback
+python3 cli.py done abc     # ValueError traceback
 ```
 
 ## Acceptance criteria
@@ -61,7 +61,7 @@ command, which already takes an index and flips a task's state; this is simply i
 
 ## Acceptance criteria
 
-- [ ] `python cli.py undone <index>` clears the done flag on the task at that index and saves.
+- [ ] `python3 cli.py undone <index>` clears the done flag on the task at that index and saves.
 - [ ] `undone` with an out-of-range or non-integer index prints a clear error and exits non-zero
       (same behavior as the fixed `done`, see Issue 1).
 - [ ] `list` after `undone` shows that task as not done (`[ ]`).