Plan: create an execution Plan from an approved Shaping doc

When the user types /plan and references an approved Shaping doc, do the following:

1) Study the Shaping doc and the codebase (required)

• Read the Shaping doc end-to-end.
• Re-identify the primary workflow(s), system boundary, and the smallest end-to-end slice.
• Search the repo for the most relevant implementation paths (models, controllers, services, jobs, GraphQL, frontend, permissions, feature flags).
• Prefer integration points that match existing patterns (Flipper flags, Sidekiq jobs, Stripe clients, GraphQL "view model" conventions, observability conventions).

2) Create a new Plan markdown file

• Location: projects/<domain-name>/<project-name>/plan.md
• 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 shaping path when possible (prefer projects/<domain-name>/<project-name>/shaping.md).
• First line: # Plan: <Project name>
• Source links: include links to the Shaping doc path (and Pitch path if available).