Add descriptions/2.18 spec (#801)

* Add API version 2.18 spec

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Add the auth tokens endpoints to the 2.18 spec

Auth Tokens joins 2.18 through the Agent Data Layer registry, so the cut's
version block doesn't list it. Copies the four operations, their schemas and
the Auth Tokens tag from Preview, with the Preview-only version header and
sentence made stable.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

* Use the placeholder token example in the 2.18 Auth Tokens spec

Main's Preview replaced the live-looking key with a placeholder in #796;
the 2.18 copy was taken from the branch's older Preview.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Vedran Zoricic <vedran.zoricic@intercom.io>

Author: 3 people

descriptions/0/api.intercom.io.yaml

@@ -53905,6 +53905,7 @@ components:
       - '2.15'
       - '2.16'
       - '2.17'
+      - '2.18'
       - Preview
     linked_object:
       title: Linked Object