Config: extract keybinding functions and clean up key configs

- Add init-kbd-func.el for keybinding helper functions
- Move global-display-line-numbers-mode from early-init.el to init.el
- Slim down init-kbd.el: remove hydra-multiple-cursors, migrate defs
- Reorganize dirvish config
- Remove scattered global-set-key bindings from init-org, init-ui, etc.
- Guard server-start with server-running-p check
- Set ibuffer-default-sorting-mode to filename
- Tweak gptel config

lisp/init-gptel.el

@@ -148,10 +148,7 @@ from the last 1500 characters of the buffer."
           (message "WARNING: gptel-request did not register callback!")))))
 
   (with-eval-after-load 'gptel
-    (define-key gptel-mode-map (kbd "C-c C-s") #'my-gptel-save-session)
-    (setf gptel--system-message
-          "You are a large language model living in Emacs and a helpful assistant. Respond concisely.
-- NEVER use unescaped org-mode headings.  Lines MUST NOT begin with `* `, `** `, `*** `, etc.  If you must start a line with asterisks, use the standard org escape format `,*` (e.g. `,* Heading`, `,** Sub-heading`).")))
+    (define-key gptel-mode-map (kbd "C-c C-s") #'my-gptel-save-session)))
 
 (use-package gptel-agent
   :init