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/23-working-with-existing-codebases/lab/orient.py

@@ -12,8 +12,8 @@ then verifies and deepens this; you never let it map from vibes alone.
 
 No dependencies. Standard library only. Works on any OS with Python 3.10+ and git.
 
-    python orient.py                 # print the pack
-    python orient.py > ORIENT.md     # save it to hand to the AI (don't commit it)
+    python3 orient.py                 # print the pack
+    python3 orient.py > ORIENT.md     # save it to hand to the AI (don't commit it)
 """
 
 from __future__ import annotations
@@ -61,7 +61,7 @@ SIGNALS: dict[str, str] = {
 
 # Common test-runner hints keyed off a present signal file.
 TEST_HINTS: dict[str, str] = {
-    "pyproject.toml": "pytest    (or: python -m pytest)",
+    "pyproject.toml": "pytest    (or: python3 -m pytest)",
     "tox.ini": "tox",
     "package.json": "npm test    (check the \"scripts\" block for the real command)",
     "go.mod": "go test ./...",