feat(acp): add steer and queue slash commands

[HenkDz]

Summary

• advertise /steer and /queue in ACP available commands
• route /steer into the existing AIAgent.steer() mechanism when a turn is active
• queue /queue prompts and regular prompts submitted during an active ACP turn
• drain queued prompts after the current turn without racing session history

Test Plan

• source venv/bin/activate && python -m pytest tests/acp_adapter/test_acp_commands.py tests/acp_adapter/test_acp_images.py tests/run_agent/test_vision_aware_preprocessing.py -q -n 4