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/06-branches-sandboxes-for-experiments/README.md

@@ -164,9 +164,9 @@ decide:
 
 ```python
 <<<<<<< HEAD
-    print("usage: python cli.py [add <title> | list | done <index> | stats]")
+    print("usage: python3 cli.py [add <title> | list | done <index> | stats]")
 =======
-    print("usage: python cli.py [add <title> | list | done <index> | purge]")
+    print("usage: python3 cli.py [add <title> | list | done <index> | purge]")
 >>>>>>> experiment
 ```
 
@@ -295,9 +295,9 @@ the one job that's still yours: verify the result.
 
    ```bash
    git diff                         # read what it actually changed
-   python cli.py add "ship module 6" --priority high
-   python cli.py add "water plants" --priority low
-   python cli.py list               # see if priorities work and sort
+   python3 cli.py add "ship module 6" --priority high
+   python3 cli.py add "water plants" --priority low
+   python3 cli.py list               # see if priorities work and sort
    ```
 
    Once the diff looks right and the feature runs, tell the agent:
@@ -312,7 +312,7 @@ the one job that's still yours: verify the result.
    > *"Switch back to `main`."*
 
    ```bash
-   python cli.py list               # no priorities; main is exactly as you left it
+   python3 cli.py list               # no priorities; main is exactly as you left it
    ```
 
    Your bold change exists only on the branch. `main` never saw it, and that's the whole point.
@@ -331,7 +331,7 @@ Then verify the result yourself:
 
 ```bash
 git log --oneline --graph            # straight line = fast-forward merge
-python cli.py list                   # the feature is now on main
+python3 cli.py list                   # the feature is now on main
 git branch                           # experiment/priorities is gone
 ```
 
@@ -343,7 +343,7 @@ Then verify:
 
 ```bash
 git log --oneline                    # no trace of the experiment on main
-python cli.py list                   # main is untouched, exactly as before
+python3 cli.py list                   # main is untouched, exactly as before
 git branch                           # the branch is gone
 ```
 
@@ -411,9 +411,9 @@ Merge conflicts have an outsized reputation for difficulty. You'll engineer a gu
 
    ```python
    <<<<<<< HEAD
-       print("usage: python cli.py [add <title> | list | done <index> | purge]")
+       print("usage: python3 cli.py [add <title> | list | done <index> | purge]")
    =======
-       print("usage: python cli.py [add <title> | list | done <index> | stats]")
+       print("usage: python3 cli.py [add <title> | list | done <index> | stats]")
    >>>>>>> feature/stats
    ```
 
@@ -446,7 +446,7 @@ Merge conflicts have an outsized reputation for difficulty. You'll engineer a gu
    should have produced a single, marker-free line listing both commands, e.g.:
 
    ```python
-       print("usage: python cli.py [add <title> | list | done <index> | stats | purge]")
+       print("usage: python3 cli.py [add <title> | list | done <index> | stats | purge]")
    ```
 
    **Here is the punchline of the whole module: you have no idea yet whether that's right, so verify.**
@@ -458,9 +458,9 @@ Merge conflicts have an outsized reputation for difficulty. You'll engineer a gu
    ```bash
    git diff HEAD~1                  # what the merge actually changed; confirm no markers remain
    git log --oneline --graph        # the fork-and-join: this is a merge commit
-   python cli.py                    # run with no args, see the merged usage string
-   python cli.py stats              # both commands actually work
-   python cli.py purge
+   python3 cli.py                    # run with no args, see the merged usage string
+   python3 cli.py stats              # both commands actually work
+   python3 cli.py purge
    ```
 
    If the usage line lists both commands and both run, the AI's silent resolution was correct. If it

modules/06-branches-sandboxes-for-experiments/lab/start/README.md

@@ -15,11 +15,11 @@ This is the running example for **Module 1** (where you feel the copy-paste prob
 ## Run it
 
 ```bash
-python cli.py add "read module 1"
-python cli.py add "set up my editor"
-python cli.py list
-python cli.py done 0
-python cli.py list
+python3 cli.py add "read module 1"
+python3 cli.py add "set up my editor"
+python3 cli.py list
+python3 cli.py done 0
+python3 cli.py list
 ```
 
 Requires Python 3.10+ (it uses `list[Task]` style type hints). No third-party packages.

modules/06-branches-sandboxes-for-experiments/lab/start/cli.py

@@ -1,8 +1,8 @@
 """Tiny command-line front end for the demo task app.
 
 Run it:
-    python cli.py add "write the lesson"
-    python cli.py list
+    python3 cli.py add "write the lesson"
+    python3 cli.py list
 
 State is kept in tasks.json next to this file. It's intentionally minimal; the point of this app
 is to be a realistic-but-small thing you change with an AI, not a product.
@@ -31,7 +31,7 @@ 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 | delete <index>]")
+        print("usage: python3 cli.py [add <title> | list | done <index> | count | delete <index>]")
         return 1
 
     command = argv[0]