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/certificates.md

@@ -292,6 +292,13 @@ jarl cert saved                            # what this host has, and when it exp
   `jarl cert get` without a name does every saved certificate. A token
   given as `@FILE` is saved as that reference, not copied. `jarl cert
   forget NAME` stops getting one (`--delete` deletes it on jarld too).
+* **Several Jarl servers on one host**: each server has its own
+  certificate names, so two servers may both have a `proxy`. The host
+  keeps one `proxy` per settings directory, though: asking another server
+  for a name saved here for the first one is refused, as is a `--dir`
+  holding another certificate's files. Choose another name or `--dir`, or
+  pass `--replace` to take the name (or directory) over. A `--dir` that
+  another saved certificate uses is always refused.
 * **The hook** runs with `sh` after new files are written, with
   `JARL_CERT_NAME`, `JARL_CERT_DIR`, `JARL_CERT_CERT`, `JARL_CERT_CHAIN`,
   `JARL_CERT_FULLCHAIN`, `JARL_CERT_KEY`, `JARL_CERT_DOMAINS`,

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