Create a chat event as the agent

Request to create a chat event.

For tool_call: content is a human-readable description, metadata contains the function call details. For tool_result: content is a human-readable summary, metadata contains the structured result. For thought: content is the agent's reasoning text. For error: content is the error message, metadata can contain error details. For task: content is the task-related message. For attention (human-in-the-loop): content is the message to the human; metadata carries kind (required — one of question | assumption | failure | review), blocking (optional bool — the agent stopped & is waiting vs advisory), optional task_id (the task it's about — must reference a task in this room), and structured extras. A resolution is a later event whose metadata.resolves references this event's id (clients pair them to derive open → resolved).

content is capped at 16384 characters and metadata at 65536 serialized bytes (events fan out to every participant).