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

modules/14-continuous-integration/README.md

@@ -78,7 +78,7 @@ Almost every CI configuration, on every forge, is the same four moves:
 4. **Run the checks**: lint, then test. Any check that exits non-zero fails the whole run.
 
 That last point is the load-bearing one. CI's entire enforcement mechanism is the **exit code**.
-Every tool you'd run in a terminal returns 0 for success and non-zero for failure. `python -m
+Every tool you'd run in a terminal returns 0 for success and non-zero for failure. `python3 -m
 unittest` exits non-zero if a test fails. `ruff check` exits non-zero if it finds a lint problem. CI runs your
 commands and watches those exit codes; one failure turns the run red. You're not learning a new
 testing system; you're wiring the tools you already have to a trigger.
@@ -154,7 +154,7 @@ When CI goes red, the skill is triage, and it's fast once you know the shape:
 3. **Read that step's log.** It's the same output the tool prints in your terminal: a failing
    `unittest` assertion, a `ruff` finding with a file and line number. CI didn't invent a new error
    format; it's showing you the command's own output.
-4. **Reproduce it locally.** The same command from the failed step (`python -m unittest` or
+4. **Reproduce it locally.** The same command from the failed step (`python3 -m unittest` or
    `ruff check .`) fails the same way on your own machine, because CI ran exactly that command. That
    reproducibility is the point: fix locally, confirm green locally, push again.
 
@@ -254,7 +254,7 @@ your machine first.
    that CI is nothing more than these same two commands is what makes the rest of the module click.
 
    ```bash
-   python -m unittest   # should report all tests passing
+   python3 -m unittest   # should report all tests passing
    ruff check .         # should report no issues (or fix what it flags)
    ```
 
@@ -325,7 +325,7 @@ and watch CI stop it.
    `git restore` (Module 12). What the agent runs looks like:
 
    ```bash
-   python -m unittest          # fails locally too: same command, same failure
+   python3 -m unittest          # fails locally too: same command, same failure
    git revert --no-edit HEAD   # new commit that undoes "Simplify pending()" (Module 12)
    git push                    # CI re-runs on the fixed code and goes green again
    ```