chore(mise): update configs and add cleanup script documentation

Update tooling configs, shell settings, and editor configurations
as part of the mise migration. Add companion documentation for the
cleanup-old-version-managers script and remove unused fish completions.

config/nvim/lua/utils.lua

@@ -102,4 +102,4 @@ function GetIntelephenseLicense()
   f:close()
   local stripped = string.gsub(content, '%s+', '')
   return stripped == '' and nil or stripped
-end
+end