gptel-org: Make gptel-org-branching-context global

* gptel-org.el (gptel-org-branching-context): Make it a global
variable as its buffer-locality is causing confusion (#764)

* NEWS: Mention change.

Author: karthink

NEWS

@@ -4,8 +4,13 @@
 
 ** Breaking changes
 
+- ~gptel-org-branching-context~ is now a global variable.  It was
+  buffer-local by default in past releases.
+
 ** New models and backends
 
+- Add support for ~gemini-2.5-pro-exp-03-25~.
+
 ** New features and UI changes
 
 - The new option ~gptel-curl-extra-args~ can be used to specify extra

gptel-org.el

@@ -143,7 +143,6 @@ heading 2.2 text
 -----
 
 This makes it feasible to have multiple conversation branches."
-  :local t
   :type 'boolean
   :group 'gptel)