framework tools/tmux: B1 round 4 — whitespace-safe hit counting; rc 4 documented

Hits newline-delimited and counted by lines (socket names with spaces
no longer break the count; codex should-fix); ambiguity listing prints
one socket per line; exit 4 added to the public contract in the header.

packages/mosaic/framework/tools/tmux/agent-send.sh

@@ -35,6 +35,9 @@
 #
 # OPTIONS
 #   -L NAME         tmux socket name passed to `tmux -L NAME` on the target host
+#
+# Exit 4: local target session exists on multiple socket servers and no
+#         -L / MOSAIC_TMUX_SOCKET disambiguated it (B1 stale-twin guard).
 #   -s DST_SESSION  target tmux session (or session:window.pane)   [required]
 #   -H SSH_TARGET   ssh target (user@host) for a remote pane; omit for local
 #   -n DST_HOST     hostname to show in the preamble for the target.
@@ -175,15 +178,16 @@ if [ -z "$SOCKET_NAME" ] && [ -z "$SSH_TARGET" ]; then
   for sf in "$socket_dir"/*; do
     [ -S "$sf" ] || continue
     sname="${sf##*/}"
-    tmux -L "$sname" has-session -t "$DST_SESSION" 2>/dev/null && hits="$hits $sname"
+    tmux -L "$sname" has-session -t "$DST_SESSION" 2>/dev/null && hits="$hits$sname"$'\n'
   done
-  hit_count=$(printf '%s' "$hits" | wc -w)
+  hit_count=$(printf '%s' "$hits" | grep -c . || true)
   if [ "$hit_count" -gt 1 ]; then
-    echo "agent-send.sh: REFUSING - session '$DST_SESSION' exists on multiple sockets:$hits" >&2
+    echo "agent-send.sh: REFUSING - session '$DST_SESSION' exists on multiple sockets:" >&2
+    printf '  %s\n' $hits >&2
     echo "  Pass -L <socket> explicitly (or export MOSAIC_TMUX_SOCKET to disambiguate)." >&2
     exit 4
   elif [ "$hit_count" -eq 1 ]; then
-    SOCKET_NAME="${hits# }"
+    SOCKET_NAME="$(printf '%s' "$hits")"
   fi
 fi