`resolve_config_env_vars` unconditionally dumped the config via
`model_dump(mode="json")` and revalidated it, which silently dropped
any field declared with `exclude=True` (e.g. `DreamConfig.cron` —
introduced by the Dream rename refactor in #2717). Result:
`agents.defaults.dream.cron` was never honored at runtime — the gateway
always fell back to the default `every 2h` schedule even when `cron`
was set in config.json.
Fix: skip the roundtrip entirely when the config has no `${VAR}`
references. Env-var interpolation still works unchanged when refs
exist; the legacy `cron` override now survives the common case of
fully-resolved config.
Regression test covers the bug path.
Allow config.json to reference environment variables via ${VAR_NAME}
syntax. Variables are resolved at runtime by resolve_config_env_vars(),
keeping the raw templates in the Pydantic model so save_config()
preserves them. This lets secrets live in a separate env file
(e.g. loaded by systemd EnvironmentFile=) instead of plain text
in config.json.
