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/13-testing-in-the-ai-era/lab/solution/reference_test_tasks.py

@@ -1,10 +1,10 @@
 """Reference test suite for the Module 13 lab. Peek only after you've tried it yourself.
 
-Named `reference_test_tasks.py` (not `test_*.py`) on purpose, so `python -m unittest discover`
+Named `reference_test_tasks.py` (not `test_*.py`) on purpose, so `python3 -m unittest discover`
 does NOT pick it up automatically. To run it, copy it next to your working `tasks.py` (e.g.
 `~/ai-workflow-course/work/tasks-app/`) and run, from that directory:
 
-    python -m unittest reference_test_tasks
+    python3 -m unittest reference_test_tasks
 
 It assumes `tasks.py` is importable, which is why you run it from the tasks-app directory.
 

modules/13-testing-in-the-ai-era/lab/start/README.md

@@ -15,11 +15,11 @@ has a `count` command (the Module 2 lab added one). The planted bug in this copy
 ## Run it
 
 ```bash
-python cli.py add "write the tests"
-python cli.py add "fix the bug"
-python cli.py done 0
-python cli.py list
-python cli.py count
+python3 cli.py add "write the tests"
+python3 cli.py add "fix the bug"
+python3 cli.py done 0
+python3 cli.py list
+python3 cli.py count
 ```
 
 Requires Python 3.10+. No third-party packages; tests use the standard library `unittest`.

modules/13-testing-in-the-ai-era/lab/start/cli.py

@@ -1,9 +1,9 @@
 """Tiny command-line front end for the demo task app.
 
 Run it:
-    python cli.py add "write the lesson"
-    python cli.py list
-    python cli.py count
+    python3 cli.py add "write the lesson"
+    python3 cli.py list
+    python3 cli.py count
 
 State is kept in tasks.json next to this file. Same minimal app from Modules 1 and 2, with a
 `count` command bolted on.
@@ -32,7 +32,7 @@ def save(tlist: TaskList) -> None:
 def main(argv: list[str]) -> int:
     tlist = load()
     if not argv:
-        print("usage: python cli.py [add <title> | list | done <index> | count]")
+        print("usage: python3 cli.py [add <title> | list | done <index> | count]")
         return 1
 
     command = argv[0]

modules/13-testing-in-the-ai-era/lab/tasks-app/README.md

@@ -15,11 +15,11 @@ has a `count` command (the Module 2 lab added one). The planted bug in this copy
 ## Run it
 
 ```bash
-python cli.py add "write the tests"
-python cli.py add "fix the bug"
-python cli.py done 0
-python cli.py list
-python cli.py count
+python3 cli.py add "write the tests"
+python3 cli.py add "fix the bug"
+python3 cli.py done 0
+python3 cli.py list
+python3 cli.py count
 ```
 
 Requires Python 3.10+. No third-party packages; tests use the standard library `unittest`.

modules/13-testing-in-the-ai-era/lab/tasks-app/cli.py

@@ -1,9 +1,9 @@
 """Tiny command-line front end for the demo task app.
 
 Run it:
-    python cli.py add "write the lesson"
-    python cli.py list
-    python cli.py count
+    python3 cli.py add "write the lesson"
+    python3 cli.py list
+    python3 cli.py count
 
 State is kept in tasks.json next to this file. Same minimal app from Modules 1 and 2, with a
 `count` command bolted on.
@@ -32,7 +32,7 @@ def save(tlist: TaskList) -> None:
 def main(argv: list[str]) -> int:
     tlist = load()
     if not argv:
-        print("usage: python cli.py [add <title> | list | done <index> | count]")
+        print("usage: python3 cli.py [add <title> | list | done <index> | count]")
         return 1
 
     command = argv[0]