fix(tools/tmux): unique per-invocation paste buffer; track auto-submit-drafts.sh

Upstream the 2026-07-09 live hot-fix from web1: send-message.sh used a fixed
tmux buffer name (__mosaic_send) shared by all concurrent senders on one
server — load-buffer overwrote load-buffer and paste-buffer -d deleted
underneath peers, cross-delivering or dropping messages. Bit the fleet during
the simultaneous restart (standing briefs swapped between sessions). Buffer
name is now unique per invocation (PID + nanoseconds).

Also brings auto-submit-drafts.sh (coordinator draft-flush watchdog, until now
only a loose file in the deployed ~/.config/mosaic/tools/tmux/) under source
control, and extends test-send-message-socket.sh with a concurrency lock:
5 parallel sends to distinct panes, asserting no drop and no cross-delivery.
The lock fails 3/3 runs against the pre-fix script and passes post-fix.

Without this, the next framework redeploy would regress the live fix.

packages/mosaic/framework/tools/tmux/test-send-message-socket.sh

@@ -47,4 +47,32 @@ if capture_default | grep -qF "agent socket hello"; then
   fail "agent-send.sh leaked named-socket message to default tmux server"
 fi
 
+# Concurrency: parallel senders on one server must not cross-deliver or drop.
+# Locks the unique-per-invocation paste buffer (a fixed buffer name raced:
+# load overwrote load, -d deleted underneath — messages swapped between panes).
+CONC_N=5
+for i in $(seq 1 "$CONC_N"); do
+  tmux -L "$SOCKET" new-session -d -s "conc-$i" -c "$TMPDIR" 'bash --noprofile --norc -i'
+done
+pids=()
+for i in $(seq 1 "$CONC_N"); do
+  "$SEND_MESSAGE" -L "$SOCKET" -t "=conc-$i" -m "CONCPAYLOAD-${i}-END" >/dev/null &
+  pids+=($!)
+done
+for pid in "${pids[@]}"; do
+  wait "$pid" || fail "concurrent send-message.sh invocation exited non-zero"
+done
+sleep 0.2
+for i in $(seq 1 "$CONC_N"); do
+  pane=$(tmux -L "$SOCKET" capture-pane -t "=conc-$i:0.0" -p)
+  printf '%s' "$pane" | grep -qF "CONCPAYLOAD-${i}-END" \
+    || fail "concurrent send dropped payload for pane conc-$i"
+  for j in $(seq 1 "$CONC_N"); do
+    [ "$j" = "$i" ] && continue
+    if printf '%s' "$pane" | grep -qF "CONCPAYLOAD-${j}-END"; then
+      fail "concurrent send cross-delivered payload $j to pane conc-$i"
+    fi
+  done
+done
+
 echo "ok - named tmux socket send tools"