Onboarding + make M15 gate catch the plant + M17 override (#6,#17,#18,#19,#29) (#58)

Co-authored-by: claude <claude@jpaul.io>
Co-committed-by: claude <claude@jpaul.io>

modules/17-secrets-config-and-environments/README.md

@@ -332,7 +332,9 @@ config per environment.
    > *"Refactor `sync.py` so it reads `TASKS_API_KEY` and `APP_ENV` from environment variables
    > instead of hardcoding them. Pick the backend URL from `APP_ENV` (dev/staging/prod). Fail loudly
    > with a clear message if `TASKS_API_KEY` is missing. Don't add any third-party dependency — load
-   > the `.env` file with a few lines of plain Python."*
+   > the `.env` file with a few lines of plain Python, and make sure the loader does **not**
+   > overwrite a variable that's already set in the environment, so a value passed on the command
+   > line still wins."*
 
    You're looking for a result shaped like this (read the diff before you accept it):
 
@@ -372,6 +374,15 @@ config per environment.
    grep -n "sk-live" sync.py     # should print nothing
    ```
 
+   **Why `setdefault` and not plain assignment?** The loader uses `os.environ.setdefault(key, value)`,
+   which sets a variable *only if it isn't already set*. That precedence is load-bearing: a value the
+   environment already supplies — like an `APP_ENV` you pass on the command line — wins over the
+   `.env` file. A loader that writes `os.environ[key] = value` instead **clobbers** anything already
+   there, so the file silently overrides your command line and Part D's override demo does nothing.
+   This matches the real-world dotenv default (`override=False`): the file fills in gaps, it doesn't
+   stomp on what's already in the environment. If the AI hands you plain assignment, that's the
+   correction to make.
+
 ### Part D — Run it from the environment
 
 5. Run it reading from your `.env`:
@@ -395,7 +406,9 @@ config per environment.
    ```
 
    Watch the backend URL change with `APP_ENV` while the source never does. That's config in the
-   environment.
+   environment. **If the URL *doesn't* change, your loader is clobbering variables that were already
+   set** — it's using `os.environ[key] = value` where it needs `os.environ.setdefault(...)` (see
+   Part C). Fix the loader so the command line wins, and the override takes effect.
 
 ### Part E — Commit, and verify the secret didn't tag along