Replace notmuch with mu4e for email

mu4e's refile moves messages between IMAP folders, enabling
cross-device sync via mbsync. notmuch tags are local-only.

- Switch from notmuch to mu/mu4e packages
- Auto-initialize mu database on first sync
- Configure mu4e with folder shortcuts and refile to Archive
- Add NixOS load-path for mu4e elisp files

home/roles/emacs/doom/init.el

@@ -176,8 +176,8 @@
        ;;zig               ; C, but simpler
 
        :email
-       ;;(mu4e +org +gmail)
-       notmuch
+       (mu4e +org)
+       ;;notmuch
        ;;(wanderlust +gmail)
 
        :app