CLI: cert get doesn't take over another server's saved name

Each Jarl server has its own certificate names, but a host keeps one
saved NAME.json (and, by default, one /etc/jarl/certs/NAME). Asking a
second server for the same name overwrote the first one's settings and
files, so the first certificate quietly stopped being renewed. Now that
is refused, as is a --dir holding another certificate's files;
--replace takes the name or directory over. A --dir another saved
certificate uses is always refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/cli.md

@@ -451,6 +451,7 @@ jarl cert export mail --dir /etc/ssl/mail            # fullchain.pem and privkey
 jarl cert export mail --part bundle                  # one part to stdout: fullchain, cert, key, bundle
 jarl cert get mail -d mail.adbraun.lol --hook 'systemctl reload postfix'   # on another host: get, download, hook
 jarl cert get                                        # every saved certificate (what the timer runs)
+jarl cert get proxy -s other --replace               # take over a name saved here for another server
 jarl cert saved                                      # this host's certificates and their expiry
 jarl cert forget mail [--delete]                     # stop getting it here
 sudo jarl cert schedule [--remove]                   # daily systemd timer for jarl cert get