fix(config): preserve excluded fields in resolve_config_env_vars

`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.

tests/config/test_env_interpolation.py

@@ -80,3 +80,25 @@ class TestResolveConfig:
 
         saved = json.loads(config_path.read_text(encoding="utf-8"))
         assert saved["channels"]["telegram"]["token"] == "${MY_TOKEN}"
+
+    def test_preserves_excluded_fields_when_no_env_refs(self, tmp_path):
+        """Regression: fields with ``exclude=True`` (e.g. DreamConfig.cron)
+        must survive ``resolve_config_env_vars`` when the config has no
+        ``${VAR}`` references. Previously the unconditional dump→revalidate
+        roundtrip silently dropped them."""
+        config_path = tmp_path / "config.json"
+        config_path.write_text(
+            json.dumps(
+                {"agents": {"defaults": {"dream": {"cron": "5 11 * * *"}}}}
+            ),
+            encoding="utf-8",
+        )
+
+        raw = load_config(config_path)
+        assert raw.agents.defaults.dream.cron == "5 11 * * *"
+
+        resolved = resolve_config_env_vars(raw)
+        assert resolved.agents.defaults.dream.cron == "5 11 * * *"
+        assert resolved.agents.defaults.dream.describe_schedule() == (
+            "cron 5 11 * * * (legacy)"
+        )