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/README.md

@@ -105,8 +105,8 @@ well-formed version of the same bug:
 
 > **Title:** `done` command crashes on an out-of-range or non-integer index
 >
-> **Context:** `python cli.py done 99` on a list with 3 tasks raises an uncaught `IndexError` and
-> dumps a traceback. `python cli.py done abc` raises `ValueError`. Either way the user sees a stack
+> **Context:** `python3 cli.py done 99` on a list with 3 tasks raises an uncaught `IndexError` and
+> dumps a traceback. `python3 cli.py done abc` raises `ValueError`. Either way the user sees a stack
 > trace instead of a helpful message.
 >
 > **Acceptance criteria:**
@@ -264,7 +264,7 @@ plenty it still can't do. Because it's carried forward across modules, skip anyt
 already built (a `delete` command, task priorities) and pick work that's genuinely still missing.
 Good candidates:
 
-1. **A bug**: `python cli.py done 99` (an out-of-range index) and `python cli.py done abc` (a
+1. **A bug**: `python3 cli.py done 99` (an out-of-range index) and `python3 cli.py done abc` (a
    non-integer) both crash with an uncaught traceback. Run them and watch.
 2. **A small, patterned feature**: an `undone <index>` command that clears a task's done flag,
    mirroring the existing `done` command (it's the inverse).