Task fan-out: create worker worktrees with pre-seeded agent sessions in one action

[liam-russell]

Problem

Farming a plan out to N parallel workers is entirely manual today, per worker: create a worktree, open its terminal, launch an agent (agent:launch), paste a prompt. For a five-subtask plan that's ~20 interactions — and nothing records which worktree belongs to which subtask.

Proposal

A fan-out action, available both as a dialog and as an MCP tool (so the planner agent in the orchestration worktree #134 can call it directly, relates #99):

Input: a list of subtasks { title, prompt, agent? } plus a base ref (typically the orchestration branch). For each subtask:

1. Create a worktree + branch off the base (needs Pick a base branch/ref when creating a worktree #135).
2. Launch the chosen agent in it — per-subtask agent/model choice, so a strong planner can dispatch cheaper workers (e.g. Sonnet).
3. Seed the initial prompt with the subtask description plus a pointer to the shared plan/steering docs on the orchestration branch.
4. Record provenance (existing worktree:listProvenance / worktree:getProvenance) linking worktree → plan → subtask, which feeds the grouped sidebar (Stacked-work view: group workers under their orchestration branch, review merges, roll-up PR #136).

UI: a confirmation step before spawning ("these 5 worktrees will be created, each launching "), then per-worktree progress. Worktree items show their subtask title.

Note: agents that run git worktree add themselves are already adopted as external worktrees — this is the managed, one-action path with provenance and seeded sessions.