Function discover_existing_key 

pub fn discover_existing_key() -> Result<Option<DiscoveredKey>, String>

Try to find an existing age key from the environment.

Checks MURK_KEY first, then reads the file at MURK_KEY_FILE if set. Does NOT read .env — for direnv users, the shim already exports both variables into the environment, so the environment is the authoritative source and .env is only a write-only convenience populated by murk init.