gptel: add custom gptel-update-destination

README: Mention `gptel-update-destination` in README.

gptel.el (gptel-update-destination, gptel--update-status,
gptel-send, gptel--insert-response): New option
`gptel-update-destination` to control how gptel's status messages
are shown.  `gptel--update-status` replaces
`gptel--update-header-line`.  Replace calls to this function
elsewhere in gptel.el.

gptel-curl.el (gptel-abort, gptel-curl--stream-cleanup,
gptel-curl--stream-insert-response): Use `gptel--update-status` in
place of `gptel--update-header-line`.

gptel-transient.el (gptel--suffix-send): Use
`gptel--update-status` in place of `gptel--update-header-line`.

README.org

@@ -344,6 +344,7 @@ Other Emacs clients for LLMs prescribe the format of the interaction (a comint s
 | =gptel-default-mode=          | Major mode for dedicated chat buffers. |
 | =gptel-prompt-prefix-alist=   | Text inserted before queries.          |
 | =gptel-response-prefix-alist= | Text inserted before responses.        |
+| =gptel-update-destination=  | Display status messages in headerline (default) or minibuffer       |
 |-----------------------------+----------------------------------------|
 
 ** COMMENT Will you add feature X?

gptel-curl.el

@@ -155,7 +155,7 @@ the response is inserted into the current buffer after point."
         (delete-process proc)
         (kill-buffer (process-buffer proc))
         (with-current-buffer buf
-          (when gptel-mode (gptel--update-header-line  " Ready" 'success)))
+          (when gptel-mode (gptel--update-status  " Ready" 'success)))
         (message "Stopped gptel request in buffer %S" (buffer-name buf)))
     (message "No gptel request associated with buffer %S" (buffer-name buf))))
 
@@ -185,7 +185,7 @@ PROCESS and _STATUS are process parameters."
               (when gptel-mode (save-excursion (goto-char tracking-marker)
                                                (insert "\n\n" (gptel-prompt-prefix-string)))))
             (with-current-buffer gptel-buffer
-              (when gptel-mode (gptel--update-header-line  " Ready" 'success))))
+              (when gptel-mode (gptel--update-status  " Ready" 'success))))
         ;; Or Capture error message
         (with-current-buffer proc-buf
           (goto-char (point-max))
@@ -210,7 +210,7 @@ PROCESS and _STATUS are process parameters."
              (t (message "ChatGPT error (%s): Could not parse HTTP response." http-msg)))))
         (with-current-buffer gptel-buffer
           (when gptel-mode
-            (gptel--update-header-line
+            (gptel--update-status
              (format " Response Error: %s" http-msg) 'error))))
       (with-current-buffer gptel-buffer
         (run-hooks 'gptel-post-response-hook)))
@@ -229,7 +229,7 @@ See `gptel--url-get-response' for details."
         (with-current-buffer (marker-buffer start-marker)
           (save-excursion
             (unless tracking-marker
-              (gptel--update-header-line " Typing..." 'success)
+              (gptel--update-status " Typing..." 'success)
               (goto-char start-marker)
               (unless (or (bobp) (plist-get info :in-place))
                 (insert "\n\n")

gptel-transient.el

@@ -418,7 +418,7 @@ will get progressively longer!"
                           (gptel--at-word-end (point)))))))
       (with-current-buffer buffer
         (setq gptel-backend backend)
-        (gptel--update-header-line " Waiting..." 'warning)
+        (gptel--update-status " Waiting..." 'warning)
         (setq position (point)))
       (setq output-to-other-buffer-p t))
      ((setq gptel-buffer-name
@@ -449,7 +449,7 @@ will get progressively longer!"
             (insert reduced-prompt))
           (setq position (point))
           (when gptel-mode
-            (gptel--update-header-line " Waiting..." 'warning))))))
+            (gptel--update-status " Waiting..." 'warning))))))
 
     (when in-place
       (setq prompt (gptel--create-prompt (point)))

gptel.el

@@ -372,6 +372,15 @@ To set the model for a chat session interactively call
           (const :tag "GPT 4" "gpt-4")
           (const :tag "GPT 4 turbo (preview)" "gpt-4-1106-preview")))
 
+(defcustom gptel-update-destination "headerline"
+  "Where update messages appear."
+  :local t
+  :safe #'gptel--always
+  :group 'gptel
+  :type '(choice
+          (const :tag "Headerline" "headerline")
+          (const :tag "Minibufer" "minibuffer")))
+
 (defcustom gptel-temperature 1.0
   "\"Temperature\" of ChatGPT response.
 
@@ -621,6 +630,16 @@ opening the file."
           (propertize msg 'face face))
     (force-mode-line-update)))
 
+(defun gptel--update-status (msg face)
+  "Update status MSG in FACE."
+  (when gptel-mode
+    (if (string= gptel-update-destination "modeline")
+        (message (propertize msg 'face face))
+      (and (consp header-line-format)
+           (setf (nth 1 header-line-format)
+                 (propertize msg 'face face))
+           (force-mode-line-update)))))
+
 (cl-defun gptel-request
     (&optional prompt &key callback
                (buffer (current-buffer))
@@ -758,7 +777,7 @@ instead."
      (list :prompt full-prompt
            :buffer gptel-buffer
            :position response-pt)))
-    (gptel--update-header-line " Waiting..." 'warning)))
+    (gptel--update-status " Waiting..." 'warning)))
 
 (defun gptel--insert-response (response info)
   "Insert RESPONSE from ChatGPT into the gptel buffer.
@@ -802,8 +821,8 @@ See `gptel--url-get-response' for details."
                   (insert response)
                   (pulse-momentary-highlight-region p (point)))
                 (when gptel-mode (insert "\n\n" (gptel-prompt-prefix-string))))
-              (when gptel-mode (gptel--update-header-line " Ready" 'success))))
+              (when gptel-mode (gptel--update-status " Ready" 'success))))
-        (gptel--update-header-line
+        (gptel--update-status
          (format " Response Error: %s" status-str) 'error)
         (message "ChatGPT response error: (%s) %s"
                  status-str (plist-get info :error)))