Add attendees to event JSON output

[tpmaerk]

Summary

• Add attendees array to event output in ekctl list events, exposing name, email, status, and role from EventKit's EKParticipant
• Empty array [] when no attendees present
• Human-readable status/role strings instead of raw enum values

Example output

"attendees": [
  {
    "name": "Jane Doe",
    "email": "jane@example.com",
    "status": "accepted",
    "role": "required"
  }
]

Closes #5