DESCRIPTION: Send a prompt to previously started subagent session.

USAGE:
1. Active session accepts steering in its current invocation.
2. Terminal session continues through new invocation of same session.
3. Can be used not only for steering, but also for discussion with subagents, obtaining additional opinions, etc.
4. Prompt MUST be written in ASD-STE100 - Simplified Technical English.

CONSTRAINTS: MUST NOT be used to solve multiple subtasks in one session. Steering should be used to clarify and adjust ONE ATOMIC subtask.