Distills the current session's task, context, and plan into a clean, copy-pasteable markdown handoff document for another agent or LLM to pick up. Captures the goal, current state, remaining plan, and the key files, decisions, and gotchas — and leaves out conversational noise, tool-call transcripts, and dead ends. Writes to `.agent/{branch}/handoff.md` and copies it to the clipboard. Use when continuing work in a fresh session, passing a task to a teammate's agent, or briefing a different model. Invoke with /handoff; add a focus phrase to scope the handoff, or --brief for a condensed version.