Spawn a separate full Claude Code session in a tmux window, never a subagent. The work gets its own context window, its own tools, and a pane the user can join.

Write the briefing to docs/spawn/<slug>.md:
- With arguments: brief that task. Without arguments: hand this conversation off, then stop working the task here; two sessions on one task collide.
- The session starts blank, so reference artifacts by path (specs, files, commits, issues) rather than pointing back to this conversation.
- Name the skills the session should run, as slash commands. Do not restate what a skill already enforces; one line ("run /debug on it, /verify before done") replaces a paragraph.
- Fence the collision zone: name the files and branches this session is using that the spawned one must not touch.
- No secrets. Keys, tokens, and credentials never enter a briefing.
Ask the user, in one round: where the session goes (a pane beside the current work, or its own window), and how much permission it gets (--dangerously-skip-permissions, or not; not means --permission-mode auto). Then spawn without stealing focus, from the project root, with the chosen flag as <perm>:
- Window: tmux new-window -d -n <slug> -c <root> 'claude <perm> "$(cat docs/spawn/<slug>.md)"'
- Pane: tmux split-window -d -h -c <root> 'claude <perm> "$(cat docs/spawn/<slug>.md)"' Outside tmux, wrap the window command in tmux new-session -d -s <slug> and tell the user to tmux attach -t <slug>.
Report where the session lives (window name, or pane in the current window) and one line on what it will do, then return to the current work. The spawned session is the user's to join, steer, or kill.

Working a draft spec (a docs/specs/<slug>.md that still has ## Open decisions): offer one spawn per open, unblocked decision. This is how the draft's decisions parallelize: drill works one decision per session, and each spawned session claims its own, so that rule holds across all of them. Claim the decision in the spec before its session spawns; the briefing carries the spec path and the decision name, nothing the spec already says.