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/10-reviewing-code-you-didnt-write/README.md

@@ -243,8 +243,8 @@ real change, then review a diff the "AI" produced and catch the trap planted in
    Then see the baseline behavior with your own eyes, because the trap is going to change it:
 
    ```bash
-   python cli.py add "write the review module"
-   python cli.py done 99        # baseline: prints "error: no task at index 99", exits non-zero
+   python3 cli.py add "write the review module"
+   python3 cli.py done 99        # baseline: prints "error: no task at index 99", exits non-zero
    echo "exit code: $?"
    ```
 
@@ -296,12 +296,12 @@ real change, then review a diff the "AI" produced and catch the trap planted in
 5. Now verify your read by running the *failure* path, not the happy one:
 
    ```bash
-   python cli.py add "a real task"
-   python cli.py delete 0        # the requested feature: works fine on the happy path
-   python cli.py add "another"
-   python cli.py done 99         # the trap: compare this to your Part A baseline
+   python3 cli.py add "a real task"
+   python3 cli.py delete 0        # the requested feature: works fine on the happy path
+   python3 cli.py add "another"
+   python3 cli.py done 99         # the trap: compare this to your Part A baseline
    echo "exit code: $?"
-   python cli.py list            # did task 99 (which doesn't exist) get marked done? did anything?
+   python3 cli.py list            # did task 99 (which doesn't exist) get marked done? did anything?
    ```
 
    In the base app, `done 99` was a clean error with a non-zero exit. After this "add a delete

modules/10-reviewing-code-you-didnt-write/lab/ai-change.patch

@@ -6,8 +6,8 @@ index 91e9276..2189230 100644
  def main(argv: list[str]) -> int:
      tlist = load()
      if not argv:
--        print("usage: python cli.py [add <title> | list | done <index>]")
-+        print("usage: python cli.py [add <title> | list | done <index> | delete <index>]")
+-        print("usage: python3 cli.py [add <title> | list | done <index>]")
++        print("usage: python3 cli.py [add <title> | list | done <index> | delete <index>]")
          return 1
  
      command = argv[0]

modules/10-reviewing-code-you-didnt-write/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 done 0
+    python3 cli.py add "write the lesson"
+    python3 cli.py list
+    python3 cli.py done 0
 
 State is kept in tasks.json next to this file. The `done` command turns a bad index into a
 clean error message and a non-zero exit code; note that behavior before you review the AI
@@ -33,7 +33,7 @@ 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>]")
+        print("usage: python3 cli.py [add <title> | list | done <index>]")
         return 1
 
     command = argv[0]

modules/10-reviewing-code-you-didnt-write/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 done 0
+    python3 cli.py add "write the lesson"
+    python3 cli.py list
+    python3 cli.py done 0
 
 State is kept in tasks.json next to this file. The `done` command turns a bad index into a
 clean error message and a non-zero exit code; note that behavior before you review the AI
@@ -33,7 +33,7 @@ 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>]")
+        print("usage: python3 cli.py [add <title> | list | done <index>]")
         return 1
 
     command = argv[0]