hermes - 💡(How to fix) Fix feat: support per-session env var overrides in plugins for multi-tenant deployments

Problem

Plugins read API keys via os.environ.get('SOME_API_KEY') at import or call time. In multi-tenant deployments, each session may need a different API key (per-brand BYO keys). Currently this requires running separate Hermes containers with different env vars — not viable when serving many tenants from one container.

Proposed Change

Allow the plugin's generate() (and is_available()) to receive a session_env: Optional[Dict[str, str]] parameter containing per-session overrides injected by the host:

def generate(self, ..., session_env: Optional[Dict[str, str]] = None) -> dict:
    api_key = (session_env or {}).get('LUMA_API_KEY') or os.environ.get('LUMA_API_KEY')

Motivation

Mother supports per-brand BYO API keys stored in brandConfig.lumaApiKey. Without per-session env injection, all brands sharing a Hermes instance must share the same system-level key.