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/01-the-copy-paste-problem/README.md

@@ -148,11 +148,13 @@ purpose** so you recognize it later.
 - Python 3.10 or newer (`python --version` or `python3 --version` to check).
 - Your usual AI chat assistant, open in a browser tab.
 
-> **One command name, the whole course through:** whichever of `python` / `python3` just printed a
-> 3.10+ version is the command to use in *every* lab from here on. The labs are written with
-> `python`; if that's "command not found" on your machine (common on current macOS and default
-> Debian/Ubuntu, where Python is installed only as `python3`), read it as `python3` (and `pip3`
-> wherever a lab uses `pip`). This note holds course-wide; we won't repeat it.
+> **One command name, the whole course through:** the labs are written with `python3`, the command
+> name current macOS and default Debian/Ubuntu actually ship (they install Python only as `python3`,
+> with no bare `python` on PATH). Run `python3 --version`; if it prints a 3.10+ version, use `python3`
+> in *every* lab from here on. If `python3` is "command not found" but `python --version` shows a
+> 3.10+ version (older or some Windows setups), read every `python3` in the labs as `python` instead.
+> Where a lab runs `pip`, use whichever pairs with your Python (`pip3` commonly goes with `python3`).
+> This note holds course-wide; we won't repeat it.
 
 ### Get the course materials
 
@@ -193,8 +195,8 @@ You now have every module's files locally, including this one's under
 3. Run it in your terminal to confirm it works:
 
    ```bash
-   python cli.py add "finish module 1"
-   python cli.py list
+   python3 cli.py add "finish module 1"
+   python3 cli.py list
    ```
 
    You should see your task listed. **This is your "real local project, an editor, and a terminal."**
@@ -205,14 +207,14 @@ You now have every module's files locally, including this one's under
 Now reproduce each failure deliberately. Keep the AI strictly in the **browser chat**; no
 editor-integrated tools yet (those arrive in Module 4). This is the "before" picture on purpose.
 
-1. **Seam 1 (multiple files).** First mark a task done so there's something to hide. Run `python
-   cli.py done 0`, then `python cli.py list` shows it as `[x]`. Now paste *only* `cli.py` into your
+1. **Seam 1 (multiple files).** First mark a task done so there's something to hide. Run `python3
+   cli.py done 0`, then `python3 cli.py list` shows it as `[x]`. Now paste *only* `cli.py` into your
    chat and ask: *"Make the `list` command hide tasks that are already done."* Apply whatever it
-   gives you and run `python cli.py list`. The clean version of this change lives in `tasks.py`, the
+   gives you and run `python3 cli.py list`. The clean version of this change lives in `tasks.py`, the
    file you *didn't* paste: open it and you'll see `render()` already owns the `[x]`/`[ ]`
    box-and-index formatting, and a `pending()` helper already returns exactly the not-done tasks. But
    the chat never saw that file, so it had to do one of two things. Either it guessed at methods it
-   couldn't see (and `python cli.py list` errors out), or it reached into the raw task list and
+   couldn't see (and `python3 cli.py list` errors out), or it reached into the raw task list and
    *re-created* that box-and-index formatting inside `cli.py`, duplicating logic that already existed
    one file over. Either way, *you* had to be the one who knew the change really belonged in the
    other file.
@@ -251,7 +253,7 @@ Be honest about the limits of this module's claims:
 
 **You're done when:**
 
-- You can run `python cli.py list` in your terminal and see output; your project, editor, and
+- You can run `python3 cli.py list` in your terminal and see output; your project, editor, and
   terminal are working together.
 - You can name the three seams where copy-paste breaks (more than one file, more than one day, no
   undo) without looking back at the lesson.