Stop handing StackPilot's own settings to the stacks it deploys (0.66.3)

docker compose ran with StackPilot's whole environment, and compose
resolves ${VAR} from the calling environment before a stack's own .env.
A stack whose compose file says SECRET_KEY: ${SECRET_KEY} therefore ran
with StackPilot's SECRET_KEY instead of the one in its .env — and with
its own again whenever it was started from a shell. AnsiblePilot hit
exactly this: credentials saved under one key became unreadable under
the other, twice.

config.child_env() is os.environ minus every StackPilot setting
(SECRET_KEY, DATA_DIR, STACKS_DIR, PORT, ...); the docker CLI and git
subprocesses now get that instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/config.py

@@ -118,3 +118,18 @@ def get_settings() -> Settings:
 
 
 settings = get_settings()
+
+
+def child_env() -> dict[str, str]:
+    """os.environ without StackPilot's own settings, for docker/git subprocesses.
+
+    Compose resolves ``${VAR}`` from the calling environment *before* the
+    stack's own ``.env``, so everything StackPilot was configured with leaked
+    into every stack it deployed. A stack whose compose file says
+    ``SECRET_KEY: ${SECRET_KEY}`` silently ran with StackPilot's key instead of
+    its own — and flipped back whenever it was started from a shell, orphaning
+    whatever it had encrypted in between. Names like SECRET_KEY, PORT or
+    DATA_DIR are far too common to share.
+    """
+    own = {name.upper() for name in Settings.model_fields}
+    return {k: v for k, v in os.environ.items() if k.upper() not in own}