[P2] Co-host management

[KalebCole]

Priority: P2 (Medium) Add, remove, and manage event co-hosts. ## Use Cases - Add a friend as co-host so they can edit - Remove co-host who is no longer helping - See who has co-host access ## Proposed Commands bash # List co-hosts partiful cohosts <eventId> # Add co-host by phone partiful cohosts <eventId> --add "+12065551234" # Add by Partiful username partiful cohosts <eventId> --add "@janedoe" # Remove co-host partiful cohosts <eventId> --remove "+12065551234" ## Output Co-hosts: Game Night ==================== 1. You (host) 2. Jane Doe (@janedoe) - added Mar 1 3. Bob Smith (+1206555...) - added Mar 5 ## API Notes From captured payload, cohostIds is an array passed during create/update. Need to discover how to add cohosts to existing event. ## Acceptance Criteria - [ ] List current co-hosts - [ ] Add co-host by phone or username - [ ] Remove co-host - [ ] Works on both create and update