Stream manual control frames

Establishes a server streaming connection that delivers manual control frames to agents using server-sent events (SSE).

This endpoint streams manual control frames, for example, for joystick movements, for a specific task to the executing agent. The agent should open this stream before reporting STATUS_EXECUTING to ensure it is ready to receive control input when the operator begins sending frames.

Each frame includes epoch and sequence metadata for handling concurrent control sessions and detecting stale or out-of-order frames. Heartbeat messages are sent periodically to maintain the connection.

The stream terminates automatically when the task reaches a terminal state (STATUS_DONE_OK or STATUS_DONE_NOT_OK).