refactor: extract gh() helper into shared util module

lua/kickstart/util.lua

@@ -0,0 +1,11 @@
+-- Shared utilities for kickstart plugin files
+local M = {}
+
+--- GitHub URL shorthand: gh('user/repo') -> 'https://github.com/user/repo'
+---@param repo string
+---@return string
+function M.gh(repo)
+  return 'https://github.com/' .. repo
+end
+
+return M