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>

gateway/run.py

@@ -93,6 +93,9 @@ if _config_path.exists():
         import yaml as _yaml
         with open(_config_path, encoding="utf-8") as _f:
             _cfg = _yaml.safe_load(_f) or {}
+        # Expand ${ENV_VAR} references before bridging to env vars.
+        from hermes_cli.config import _expand_env_vars
+        _cfg = _expand_env_vars(_cfg)
         # Top-level simple values (fallback only — don't override .env)
         for _key, _val in _cfg.items():
             if isinstance(_val, (str, int, float, bool)) and _key not in os.environ: