fix(#808): never borrow a session identity for non-tmux senders (#1335)

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

@@ -126,7 +126,16 @@ if [ -z "$SRC_LABEL" ]; then
   src_host=$(hostname -s 2>/dev/null || echo "?")
   src_sess=${MOSAIC_AGENT_NAME:-}
   if [ -z "$src_sess" ]; then
-    src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    if [ -n "${TMUX:-}" ]; then
+      # Inside tmux: display-message resolves against this client's own session.
+      src_sess=$(tmux display-message -p '#S' 2>/dev/null || echo "?")
+    else
+      # Outside tmux with no name: display-message reports the LAST-ACTIVE
+      # session — someone else's identity (measured 2026-08-20: a nameless
+      # non-tmux sender was stamped "peggy", a live seat, forged silently).
+      # Stamp an explicit unverified label instead; deliberate senders use -S.
+      src_sess="unverified"
+    fi
   fi
   SRC_LABEL="${src_host}:${src_sess}"
 fi