Breakdown: convert a Plan into executable tasks

When the user types /breakdown and references a Plan doc, do the following:

1) Study the Plan and the codebase (required)

• Read the Plan end-to-end.
• Identify the "must ship" scope, milestones, and the critical path.
• Search the repo for the most relevant implementation paths named in the Plan and confirm they exist.
• Prefer tasks that match existing repo patterns (Flipper flags, Sidekiq jobs, Stripe clients, GraphQL view-model discipline, observability conventions).

2) Create individual task files

• Location: projects/<domain-name>/<project-name>/tasks/
• Domain name: the product area or team domain (e.g., payments, family-messaging, attendance-plus)
• Project name: derive a stable folder slug from the referenced Plan (prefer projects/<domain-name>/<project-name>/plan.md).
• Filename: <milestone>-<task>-<short-slug>.md (e.g., 1-1-spike-payout-composition.md, 2-1-add-stripe-payout-model.md, 2-2-add-stripe-payout-item.md)
• First line: # <milestone>-<task>: <Task title> (e.g., # 1-1: Spike payout composition, # 2-1: Add Stripe payout model)
• Source link: reference the Plan doc path (and shaping/pitch paths if present in the Plan).