fix: remove sidebar.close_from_input default keybinding (#1536)

* Remove <c-d> from default insert mode keybinding

* Check `close_from_input ~= nil` before keymap

* Default `close_from_input` to `nil`

* Update README.md keybinding defaults

* Try to pass type check

lua/avante/config.lua

@@ -392,10 +392,7 @@ M._defaults = {
       remove_file = "d",
       add_file = "@",
       close = { "<Esc>", "q" },
-      close_from_input = {
-        normal = "<Esc>",
-        insert = "<C-d>",
-      },
+      close_from_input = nil, -- e.g., { normal = "<Esc>", insert = "<C-d>" }
     },
     files = {
       add_current = "<leader>ac", -- Add current buffer to selected files