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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.66.2",
+  "version": "0.66.3",
   "type": "module",
   "scripts": {
     "dev": "vite",