Add app subscription migration commands

[tyler-eon]

Why

Partners migrating legacy manual-billing subscriptions to Shopify-managed app pricing need a safe, scriptable CLI workflow.

What changed

Adds four core Shopify CLI commands:

• shopify app subscription-migrations schedule
• shopify app subscription-migrations unschedule
• shopify app subscription-migrations status
• shopify app subscription-migrations cancel

The commands:

• resolve the app Client ID from the active app configuration, with an optional --client-id override;
• accept migration CSVs through --input, stdin, or --input -;
• validate the complete CSV before making a network mutation;
• normalize and sort shop GIDs, then create deterministic batches of at most 250 shops;
• generate a portable root idempotency key and deterministic per-batch keys;
• preserve every accepted operation GID when a later batch fails;
• support repeated --id flags for multi-batch status and cancellation;
• show visible polling progress for watched submissions and status checks;
• keep JSON stdout machine-readable while polling progress is rendered on stderr.

cancel stops an operation from processing additional shops. It does not undo shops that have already been scheduled or migrated; unschedule is the separate reversal workflow.

Testing

• pnpm vitest run packages/app/src/cli — 2,250 tests passed
• focused subscription migration suite — 157 tests passed
• pnpm --filter @shopify/app lint
• pnpm --filter @shopify/app type-check
• pnpm --filter @shopify/app build
• CLI command discovery and help for all four commands
• command-tree snapshot verification
• malformed file/stdin CSV validation before authentication or network access
• generated README, oclif manifest, and Shopify.dev command documentation verification