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/21-skills-teaching-the-ai-your-playbook/lab/add-command-skill.md

@@ -24,7 +24,7 @@ Ask for these if they weren't given:
 
 - Core logic lives in `tasks.py` (the `TaskList` class). The CLI front end is `cli.py`. State
   persists to `tasks.json`. **Never edit `tasks.json` by hand; it's generated.**
-- Tests live in `test_tasks.py` and run with `python -m unittest`. Standard library only; no
+- Tests live in `test_tasks.py` and run with `python3 -m unittest`. Standard library only; no
   third-party packages, no new dependencies.
 - The human-facing change log is `CHANGELOG.md`, newest entry on top.
 
@@ -42,10 +42,10 @@ Ask for these if they weren't given:
    crash." Assert the end state (e.g. after `clear()`, `len(tasks) == 0` and `pending()` is empty).
    A test that passes against a broken implementation is worse than no test.
 
-4. **Run the tests.** `python -m unittest` from the project root. Do not claim success until it's
+4. **Run the tests.** `python3 -m unittest` from the project root. Do not claim success until it's
    green. If it fails, fix the code, not the test, and run again.
 
-5. **Smoke-test the CLI.** Actually run it: `python cli.py COMMAND_NAME`, then `python cli.py list`
+5. **Smoke-test the CLI.** Actually run it: `python3 cli.py COMMAND_NAME`, then `python3 cli.py list`
    to confirm the visible result. Paste what you ran and what it printed.
 
 6. **Add a `CHANGELOG.md` entry.** One line under the top heading, present tense:
@@ -57,8 +57,8 @@ Ask for these if they weren't given:
 
 ## Done when
 
-- `python -m unittest` is green and includes a new test that actually exercises `COMMAND_NAME`.
-- `python cli.py COMMAND_NAME` does `WHAT_IT_DOES` and you've shown the output.
+- `python3 -m unittest` is green and includes a new test that actually exercises `COMMAND_NAME`.
+- `python3 cli.py COMMAND_NAME` does `WHAT_IT_DOES` and you've shown the output.
 - `CHANGELOG.md` has a new top line for the command.
 - One commit contains the code, the test, and the changelog line, and nothing else (no
   `tasks.json`, no unrelated reformatting).

modules/21-skills-teaching-the-ai-your-playbook/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. The same minimal app from Module 1 onward; the
 target your "add a command" skill extends.
@@ -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/21-skills-teaching-the-ai-your-playbook/lab/start/test_tasks.py

@@ -1,6 +1,6 @@
 """Test suite for the tasks-app. Run from this folder with:
 
-    python -m unittest
+    python3 -m unittest
 
 Your "add a command" skill should ADD a test here for every new command. The point is to assert
 intended behavior, not just that nothing crashed.

modules/21-skills-teaching-the-ai-your-playbook/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. The same minimal app from Module 1 onward; the
 target your "add a command" skill extends.
@@ -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/21-skills-teaching-the-ai-your-playbook/lab/tasks-app/test_tasks.py

@@ -1,6 +1,6 @@
 """Test suite for the tasks-app. Run from this folder with:
 
-    python -m unittest
+    python3 -m unittest
 
 Your "add a command" skill should ADD a test here for every new command. The point is to assert
 intended behavior, not just that nothing crashed.