pkgs/clan: Improve error message on clan secrets users add

Improve error message on `clan secrets users add [user] --age-key
`AGE-PLUGIN-YUBIKEY`

Since there is no way to get the recipient through the AGE-PLUGIN-YUBIKEY,
we should fail fast and give an actionable error to the user.

pkgs/clan-cli/clan_cli/secrets/types.py

@@ -21,6 +21,12 @@ def secret_name_type(arg_value: str) -> str:
 def public_or_private_age_key_type(arg_value: str) -> str:
     if Path(arg_value).is_file():
         arg_value = Path(arg_value).read_text().strip()
+    elif arg_value.startswith("AGE-PLUGIN-"):
+        msg = (
+            f"AGE-PLUGIN keys cannot be used directly as they are plugin identifiers, not recipient keys. "
+            f"Please provide the corresponding age1 public key instead. Got: '{arg_value}'"
+        )
+        raise ClanError(msg)
 
     public_keys = get_public_age_keys(arg_value)