feat: add agent participants to private chats

[ibourgeois]

Summary

Allow a private chat to include one or more agent participants.

Why This Matters

Katra needs a durable way to model that a conversation is with an agent rather than only with human users. That should exist before building the first real agent chat loop.

Acceptance Criteria

• add an agent participant model or equivalent relationship for chats
• a chat can include human users, agent participants, or both
• the participant model makes it clear which side is user-owned versus agent-owned
• the current UI can render enough participant metadata to distinguish an agent chat from a human-only chat

Notes

This issue is about participant identity and relationships, not message execution or response generation.