Primary: /run/agenix/elevenlabs-key (read via 'read -r' which strips
the trailing newline agenix files contain).
Fallback: ELEVENLABS_API_KEY env var (backwards compatible).
No need to export the key in shell environment anymore — if the
agenix secret exists, talk picks it up automatically.
Override path via ELEVENLABS_KEY_FILE env var if needed.