docs(shopify): document Protected Customer Data requirement

[guitavano]

Follow-up to #526.

Using the customer tools (SHOPIFY_LIST_CUSTOMERS, lookups, segments, order→customer joins) fails with:

This app is not approved to access the Customer object. See https://shopify.dev/docs/apps/launch/protected-customer-data …

even with the read_customers scope granted. That's Shopify's Protected Customer Data gate — a separate, app-level approval independent of OAuth scopes. It's a Partner-dashboard configuration, not a code issue (the MCP already surfaces the error with a clear hint).

Adds a README section covering:

• what the gate is and the exact error,
• the Partner Dashboard steps to grant it (level 1 protected data + level 2 PII fields, questionnaire, reconnect),
• the option to drop read_customers from SHOPIFY_SCOPES if customer data isn't needed.

Docs-only — no code or registry.json change (the registry readme comes from mesh_description, not README.md).

🤖 Generated with Claude Code

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Summary by cubic

Add README guidance for Shopify’s Protected Customer Data so customer tools work and the “not approved to access the Customer object” error is avoided. Also notes how to remove read_customers if customer data isn’t needed.

• Migration
  • Partner Dashboard → Apps → your app → API access → request “Protected customer data” (level 1) and, if needed, “Protected customer fields” (level 2).
  • Complete the questionnaire, then re-run OAuth to reconnect.
  • If not using customer data, remove read_customers from SHOPIFY_SCOPES.

^{Written for commit 63be6e8. Summary will update on new commits.}