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/12-revert-reset-and-recovery/README.md

@@ -295,9 +295,9 @@ that *you* hold the judgment: which undo, which parent, whether it actually work
 4. **Now feel the bug.** It passes the first skim:
 
    ```bash
-   python cli.py add "ship it"
-   python cli.py clear          # prints "cleared all tasks", looks fine!
-   python cli.py list           # CRASHES: it corrupted tasks.json, load() blows up
+   python3 cli.py add "ship it"
+   python3 cli.py clear          # prints "cleared all tasks", looks fine!
+   python3 cli.py list           # CRASHES: it corrupted tasks.json, load() blows up
    ```
 
    This is the AI plausibility trap made concrete: the change reviewed fine and "worked," and broke
@@ -337,8 +337,8 @@ that *you* hold the judgment: which undo, which parent, whether it actually work
 
    ```bash
    rm -f tasks.json                              # drop the corrupted state file the bug wrote
-   python cli.py add "back to normal"
-   python cli.py list                            # works again, the clear command is gone
+   python3 cli.py add "back to normal"
+   python3 cli.py list                            # works again, the clear command is gone
    git log --oneline                             # the bad merge is STILL there, with a revert after it
    ```
 
@@ -360,7 +360,7 @@ that *you* hold the judgment: which undo, which parent, whether it actually work
 
    ```bash
    git log --oneline -1         # "Add version command"
-   python cli.py version        # prints the version
+   python3 cli.py version        # prints the version
    ```
 
 2. Now destroy it the way an over-eager "clean up the history" cleanup (or an agent) would, with a
@@ -369,7 +369,7 @@ that *you* hold the judgment: which undo, which parent, whether it actually work
    ```bash
    git reset --hard HEAD~1
    git log --oneline -2         # the "Add version command" commit is GONE from the branch
-   python cli.py version 2>/dev/null || echo "command no longer exists"
+   python3 cli.py version 2>/dev/null || echo "command no longer exists"
    ```
 
    It's not in `log`. It feels permanently lost. It isn't.
@@ -384,7 +384,7 @@ that *you* hold the judgment: which undo, which parent, whether it actually work
 
    ```bash
    git log --oneline -1         # "Add version command" is back
-   python cli.py version        # works again
+   python3 cli.py version        # works again
    ```
 
    You just recovered a commit that `log` swore was gone. Note the honest limit: step 2's `--hard`