Post to the canonical conversation

Requires an active organization membership and threads:write scope. Persists the authenticated sender, validates mention targets and notifies human mentions through Inbox. Each distinct mentioned agent gets a durable queued thread in the channel's default environment, falling back to the organization default. Missing environments return channel_environment_required (409); unavailable environments or agents return 404. Custom agents require a published version (channel_agent_not_published, 409); its version is pinned at admission. Message, thread associations and jobs commit atomically. Retain idempotencyKey for retries; changed content returns 409 without starting another run. Archived channels reject posting. Execution rechecks membership, resource access, capacity, budget and runtime permissions; final replies are posted back to the channel. Optional threadIds share up to 20 owned, nondeleted threads from this organization. Sending atomically grants current channel members read access to the original threads and includes them in the channel Threads list. It does not grant write or reshare rights, move originals, or enqueue work without agent mentions. Unavailable or unowned threads return channel_thread_unavailable (404). Content may be empty for a thread share. Message metadata.threadAttachments contains server-resolved id and title pairs. Retries reuse the original event.