feat: add Fin Agent API orchestration endpoints to Preview spec (#541)

Add the Fin Agent API orchestration operations to the Preview spec:
discover capabilities, ask a one-shot question, reply, run a procedure,
and escalate to a human, alongside the existing conversational start.

Co-authored-by: Ailish McCarthy <ailish.mccarthy@intercom.io>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people