feat: add Squarespace Commerce integration

[pranavrajs]

Adds the Squarespace Commerce integration (Commerce). Look up Squarespace customers, orders, shipment tracking, and product stock.

Tools

• Find Customers (find_customers, POST)
• List Customer Orders (list_customer_orders, GET)
• Get Order (get_order, GET)
• List Products (list_products, GET)
• Get Product (get_product, GET)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from Squarespace Commerce'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

• API keys need the Commerce Advanced plan.
• Orders can only be looked up by customer ID, so guest orders with no customer record will not appear.
• Sets a fixed User-Agent header because Squarespace rejects requests without one.

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.