add copilot support

codex.el

@@ -71,6 +71,17 @@
   :group 'codex)
 
 (defvar codex--session-id nil)
+(defvar codex--use-copilot nil
+  "In order for copilot integration to work, you need a section in your
+~/.codex/config.toml like this:
+[model_providers.github-copilot]
+name = \"Github Copilot\"
+base_url = \"https://api.githubcopilot.com\"
+env_key = \"GITHUB_COPILOT_TOKEN\"
+wire_api = \"chat\"
+http_headers = {
+    Copilot-Integration-Id = \"vscode-chat\"
+}")
 
 (defvar codex-mode-map
   (let ((map (make-sparse-keymap)))
@@ -208,12 +219,14 @@
                 :name "codex"
                 :buffer buf
                 :command
-                (let ((resume (when (and codex--session-id (not (string-empty-p codex--session-id)))
-                               (progn
-                                 (message "Using session %s" codex--session-id)
-                                 (list "resume" codex--session-id))))
-                      (skip (and codex-skip-git-repo-check (list "--skip-git-repo-check"))))
-                  (append (list codex-command "exec") resume skip (list "--json" prompt))))))
+                (let* ((resume (when (and codex--session-id (not (string-empty-p codex--session-id)))
+                                (progn
+                                  (message "Using session %s" codex--session-id)
+                                  (list "resume" codex--session-id))))
+                       (skip (and codex-skip-git-repo-check (list "--skip-git-repo-check")))
+                       (copilot (and codex--use-copilot (list "-c" "model_provider=github-copilot" "-m" "claude-haiku-4.5")))
+                       (command (append (list codex-command "exec") resume skip copilot (list "--json" prompt))))
+                  command))))
     (set-process-sentinel
      proc
      (lambda (p event)