feat: add Square integration

[pranavrajs]

Adds the Square integration (Payments & billing). Check customer orders, payments, refunds, invoices, and subscriptions in Square.

Tools

• Find Customers by Email (find_customers_by_email, POST)
• List Orders (list_orders, POST)
• Get Payment (get_payment, GET)
• Get Refund (get_refund, GET)
• List Invoices (list_invoices, POST)
• List Subscriptions (list_subscriptions, POST)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from Square's API docs.
• Not run against the live API. No credentials were available, so tool-tester.rb has not been run with a real key. This needs doing before merge.
• No logo is included.

Review notes

• Square's personal access token is full-access.
• Orders and invoices are searched at one location, set at install.

The root README.md row and .env.example entry for this integration are in a separate PR, so the per-integration PRs do not conflict with each other.