feat: add Webflow integration

[pranavrajs]

Adds the Webflow integration (Commerce). Look up e-commerce orders, products, and published CMS items on a Webflow site.

Tools

• Get Order (get_order, GET)
• List Recent Orders (list_recent_orders, GET)
• List Orders by Status (list_orders_by_status, GET)
• List Products (list_products, GET)
• List Collections (list_collections, GET)
• List Collection Items (list_collection_items, GET)
• Find Collection Item by Slug (find_collection_item_by_slug, GET)

Testing

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

• Webflow's orders API has no email filter, so there is no find-by-email tool; Captain needs the order ID or pages through orders.
• SKU prices are divided by 100, which is wrong for zero-decimal currencies.
• Webflow recommends OAuth for public or multi-site apps but supports site tokens for single-site integrations.

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.

[pranavrajs]

Closing: no new integrations are being added in this round. The branch is kept in case this is wanted later.