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

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