Codex-style /btw command for DeepSeek Harness

Key features

• The command is registered through @deepseek-ai/dsh-commands (the same registry that powers /compact, /goal, /plan).
• The question is delegated through ctx.subagents.start("fork", …).
• The fork provider spawns a child agent seeded with the parent's completed turns, so the side question is answered with full knowledge of the main conversation.
• The child's answer is returned as a plain command result.
• Command results are rendered by the UI adapter and never enter the model history — that is the "no pollution" guarantee.