refactor: extract resolve_joinable_party from join and knock

cmd_join and cmd_knock both resolved an optional name argument to a
live party via find_party_by_name/pick_live_party with identical error
handling. Share it in one helper.

party

@@ -635,6 +635,26 @@ pick_live_party() {
     eval "roster_read \"\$$pick\""
 }
 
+# Resolve the party a join or knock targets: the named party if given,
+# else the single live party (or a numbered prompt via pick_live_party).
+# Sets RR_* on success; exits 1 with the appropriate message otherwise.
+resolve_joinable_party() {
+    _rj_name="$1"
+    if [ -n "$_rj_name" ]; then
+        # Capture rc explicitly: under set -e a bare `find_party_by_name`
+        # returning 1 or 2 would exit before the case ran, swallowing the
+        # error message.
+        find_party_by_name "$_rj_name" && rc=0 || rc=$?
+        case $rc in
+            0) ;;
+            1) echo "party: '$_rj_name' not found." >&2; exit 1 ;;
+            2) exit 1 ;;   # find_party_by_name already printed the ambiguity hint
+        esac
+    else
+        pick_live_party || exit 1
+    fi
+}
+
 # Subcommand dispatch
 # ===================
 
@@ -1189,19 +1209,7 @@ EOF
         exit 1
     fi
 
-    if [ -n "$name" ]; then
+    resolve_joinable_party "$name"
-        # Capture rc explicitly: under set -e a bare `find_party_by_name`
-        # returning 1 or 2 would exit before the case ran, swallowing the
-        # error message.
-        find_party_by_name "$name" && rc=0 || rc=$?
-        case $rc in
-            0) ;;
-            1) echo "party: '$name' not found." >&2; exit 1 ;;
-            2) exit 1 ;;   # find_party_by_name already printed the ambiguity hint
-        esac
-    else
-        pick_live_party || exit 1
-    fi
 
     # Group gate. Check membership against the party's *recorded* group
     # (RR_GROUP), the host may have hosted under --group <other>. The
@@ -1355,16 +1363,7 @@ EOF
         esac
     done
 
-    if [ -n "$name" ]; then
+    resolve_joinable_party "$name"
-        find_party_by_name "$name" && rc=0 || rc=$?
-        case $rc in
-            0) ;;
-            1) echo "party: '$name' not found." >&2; exit 1 ;;
-            2) exit 1 ;;   # find_party_by_name printed the ambiguity hint
-        esac
-    else
-        pick_live_party || exit 1
-    fi
 
     if [ "$RR_HOST_USER" = "$USER" ]; then
         echo "party knock: '$RR_PARTY_NAME' is your own party." >&2