Create schedule

The action is derived from the shape: agent_id + message creates an agent_run task that fires as a durable agent run carrying the guardrail-screened message (endpoint_id/payload are rejected); agent_id without message is the pre-46a compatibility shape, stored as agent_run with the exact message scheduled trigger and its legacy delivery fields retained but never executed; message + model (+ optional tools) with NO agent_id creates an agent_run task for which the engine makes a hidden personal agent — persona framed around the message, the named tools at their default policy — and binds it (the response carries its agent_id; the agent is deleted with the schedule; model is required because the engine has no organization default; model/tools are rejected on every other shape); no agent_id and no message creates the original webhook action, which requires endpoint_id. Responses expose action_type, the stored message (agent_run only), and the engine-computed owned_by_caller.