feat: add beehiiv integration

[pranavrajs]

Adds the beehiiv integration (Marketing). Look up newsletter subscribers, their subscription status and tier, recent posts, and paid tiers in beehiiv, and tag subscribers.

Tools

• Find Subscriber by Email (find_subscriber_by_email, GET)
• Get Subscription (get_subscription, GET)
• List Recent Posts (list_recent_posts, GET)
• List Tiers (list_tiers, GET)
• Add Subscription Tag (add_subscription_tag, POST)

Testing

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

• Add Subscription Tag is the one write tool, and adding a tag can trigger the publication's own automations.
• Find Subscriber by Email uses the list endpoint with an email filter, so an unknown email returns an empty list instead of a 404.
• The README setup steps come from beehiiv's help article; the developer docs page for key creation could not be fetched.

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.