Returns the organization merge / piping variable catalog for composing
templated copy across messages, sequences, agents, campaigns, and forms.

Without form_id: form-safe and message-only builtins (tagged with
products) plus custom-object properties as
{{custom.<object>.<property>:contact}}.

API keys receive the full catalog. OAuth tokens receive custom-object
tokens only with the custom_data_types:read scope and matching
workspace permission, and data.form only with forms:read. Builtins
are always included. A missing scope omits that section; it does not
reject the request.

With form_id (pass after the form has elements — create or draft): also
returns data.form with concrete {{answers.<element_id>}},
{{params.*}}, {{vars.*}}, and {{steps.<http-id>.<var>}} tokens plus
form syntax notes. Form branching on prior answers uses logic rules
(trigger_element_id + conditions → jump_to / end_form / set_variable),
not this catalog — see GET /v1/forms/field-types logic_rule_shape.

A malformed form_id (not a UUID) returns 422 when the caller can read
forms. Copy tokens exactly — never invent custom paths.