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.

nanobot/config/loader.py

@@ -78,6 +78,9 @@ def save_config(config: Config, config_path: Path | None = None) -> None:
         json.dump(data, f, indent=2, ensure_ascii=False)
 
 
+_ENV_REF_PATTERN = re.compile(r"\$\{([A-Za-z_][A-Za-z0-9_]*)\}")
+
+
 def resolve_config_env_vars(config: Config) -> Config:
     """Return a copy of *config* with ``${VAR}`` env-var references resolved.
 
@@ -85,14 +88,28 @@ def resolve_config_env_vars(config: Config) -> Config:
     Raises :class:`ValueError` if a referenced variable is not set.
     """
     data = config.model_dump(mode="json", by_alias=True)
+    if not _has_env_refs(data):
+        # Skip the dump→revalidate roundtrip so fields with ``exclude=True`` survive.
+        return config
     data = _resolve_env_vars(data)
     return Config.model_validate(data)
 
 
+def _has_env_refs(obj: object) -> bool:
+    """Return True if any string value contains a ``${VAR}`` reference."""
+    if isinstance(obj, str):
+        return bool(_ENV_REF_PATTERN.search(obj))
+    if isinstance(obj, dict):
+        return any(_has_env_refs(v) for v in obj.values())
+    if isinstance(obj, list):
+        return any(_has_env_refs(v) for v in obj)
+    return False
+
+
 def _resolve_env_vars(obj: object) -> object:
     """Recursively resolve ``${VAR}`` patterns in string values."""
     if isinstance(obj, str):
-        return re.sub(r"\$\{([A-Za-z_][A-Za-z0-9_]*)\}", _env_replace, obj)
+        return _ENV_REF_PATTERN.sub(_env_replace, obj)
     if isinstance(obj, dict):
         return {k: _resolve_env_vars(v) for k, v in obj.items()}
     if isinstance(obj, list):

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)"
+        )