Duplicate an agent-calendar link (virtual calendar)

Creates a second AgentCalendar row pointing at the same underlying calendar but with independent duration/description. Use this to expose multiple "virtual" variants of one Google/Outlook/Apple calendar to the AI — e.g., "New patient — 40 min" and "Follow-up — 10 min" on the same physical calendar. The AI sees them as distinct options and picks one per appointment type. Only allowed for self-managed providers (GOOGLE, OUTLOOK, APPLE); external booking engines (Calendly, Cal.com, Square, GHL) dictate duration from their own system, so cloning is rejected. :agentCalendarId is the AgentCalendar primary key (not the underlying calendar's ID).