Use python3 as the canonical command name course-wide (#104) (#105)

modules/02-version-control-as-a-safety-net/README.md

@@ -204,7 +204,7 @@ and your AI assistant.
 
    This is the habit that replaces "paste it back and hope." You're reading exactly what changed,
    nothing more, nothing less. Confirm it does what you asked and didn't touch anything it shouldn't.
-   Run it (`python cli.py count`), then commit:
+   Run it (`python3 cli.py count`), then commit:
 
    ```bash
    git add .
@@ -223,7 +223,7 @@ and your AI assistant.
    git status                 # shows tasks.py as modified
    git restore tasks.py       # discard the change; back to your last commit, byte for byte
    git diff                   # empty: nothing changed. you're clean.
-   python cli.py list         # works again
+   python3 cli.py list         # works again
    ```
 
    You just recovered from a bad AI change in one command, with zero retyping and zero guesswork.
@@ -258,7 +258,7 @@ and your AI assistant.
 
 9. Close the loop and leave the repo clean. The cold session just told you what's in progress and
    what to do next: finish the `delete <index>` command. Do that with the AI (paste in `cli.py` the
-   same way as Part B), run it to confirm it works (`python cli.py delete 1`), then commit:
+   same way as Part B), run it to confirm it works (`python3 cli.py delete 1`), then commit:
 
    ```bash
    git add .

modules/02-version-control-as-a-safety-net/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/02-version-control-as-a-safety-net/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>]")
+        print("usage: python3 cli.py [add <title> | list | done <index>]")
         return 1
 
     command = argv[0]