feat(config): support ${ENV_VAR} substitution in config.yaml (#2684)

* feat(config): support ${ENV_VAR} substitution in config.yaml

* fix: extend env var expansion to CLI and gateway config loaders

The original PR (#2680) only wired _expand_env_vars into load_config(),
which is used by 'hermes tools' and 'hermes setup'. The two primary
config paths were missed:

- load_cli_config() in cli.py (interactive CLI)
- Module-level _cfg in gateway/run.py (gateway — bridges api_keys to env vars)

Also:
- Remove redundant 'import re' (already imported at module level)
- Add missing blank lines between top-level functions (PEP 8)
- Add tests for load_cli_config() expansion

---------

Co-authored-by: teyrebaz33 <hakanerten02@hotmail.com>

cli.py

@@ -301,7 +301,11 @@ def load_cli_config() -> Dict[str, Any]:
                 defaults["agent"]["max_turns"] = file_config["max_turns"]
         except Exception as e:
             logger.warning("Failed to load cli-config.yaml: %s", e)
-    
+
+    # Expand ${ENV_VAR} references in config values before bridging to env vars.
+    from hermes_cli.config import _expand_env_vars
+    defaults = _expand_env_vars(defaults)
+
     # Apply terminal config to environment variables (so terminal_tool picks them up)
     terminal_config = defaults.get("terminal", {})