say: default provider to Jeffrey PVC
`say hello` (no colon prefix) now speaks in @jeffrey's cloned voice.
Other providers stay one colon away:
say:openai / say:google / say:eleven / say:jeffrey (explicit)
Both the `say.mjs` piece and the `/api/say` backend share the default,
so direct API calls without a `provider` field also land on Jeffrey.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>