fix: standalone setup — garage config, symlink handling, healthcheck

- garage.toml: fix rpc_secret field name (was secret_transmitter),
  move to top-level per Garage v1.1.0 spec, remove unused [s3_web]
- setup-standalone.sh: resolve symlinked .env files before writing,
  always ensure all standalone-critical vars via env_set,
  fix garage key create/info syntax (positional arg, not --name),
  avoid overwriting key secret with "(redacted)" on re-run,
  use compose_cmd in health check
- docker-compose.standalone.yml: fix garage healthcheck (no curl in
  image, use /garage stats instead)

docker-compose.standalone.yml

@@ -17,7 +17,7 @@ services:
       - ./data/garage.toml:/etc/garage.toml:ro
     restart: unless-stopped
     healthcheck:
-      test: ["CMD", "curl", "-sf", "http://localhost:3903/health"]
+      test: ["CMD", "/garage", "stats"]
       interval: 10s
       timeout: 5s
       retries: 5