feat: add Teachable integration

[pranavrajs]

Adds the Teachable integration (Commerce). Look up students, course enrollments, progress, purchases, and pricing plans in a Teachable school.

Tools

• Find User by Email (find_user_by_email, GET)
• Get User Enrollments (get_user_enrollments, GET)
• Get Course Progress (get_course_progress, GET)
• List Transactions (list_transactions, GET)
• Get Pricing Plan (get_pricing_plan, GET)
• Find Courses (find_courses, GET)

Testing

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

• Transaction amounts are shown raw and labelled as the smallest currency unit; the docs state that unit only for pricing plans.
• The transactions list documents no sort parameter, so "most recent" is not guaranteed.
• The API needs a Teachable plan that includes API access.

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.