docs: plan durable tmux fleet install (#557)

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

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+SEND_MESSAGE="$SCRIPT_DIR/send-message.sh"
+AGENT_SEND="$SCRIPT_DIR/agent-send.sh"
+SOCKET="mosaic-test-$RANDOM-$$"
+TARGET="target-$RANDOM"
+DEFAULT_TARGET="default-target-$RANDOM"
+TMPDIR=$(mktemp -d)
+trap 'tmux -L "$SOCKET" kill-server >/dev/null 2>&1 || true; tmux kill-session -t "$DEFAULT_TARGET" >/dev/null 2>&1 || true; rm -rf "$TMPDIR"' EXIT
+
+fail() {
+  echo "FAIL: $*" >&2
+  exit 1
+}
+
+require_tmux() {
+  command -v tmux >/dev/null 2>&1 || fail "tmux is required"
+}
+
+capture_named() {
+  tmux -L "$SOCKET" capture-pane -t "=$TARGET:0.0" -p
+}
+
+capture_default() {
+  tmux capture-pane -t "=$DEFAULT_TARGET:0.0" -p
+}
+
+require_tmux
+
+tmux -L "$SOCKET" new-session -d -s "$TARGET" -c "$TMPDIR" 'bash --noprofile --norc -i'
+tmux new-session -d -s "$DEFAULT_TARGET" -c "$TMPDIR" 'bash --noprofile --norc -i'
+
+"$SEND_MESSAGE" -L "$SOCKET" -t "=$TARGET" -m "named socket hello" >/tmp/send-message-named.out
+sleep 0.2
+capture_named | grep -qF "named socket hello" || fail "send-message.sh did not deliver to named socket"
+if capture_default | grep -qF "named socket hello"; then
+  fail "send-message.sh leaked named-socket message to default tmux server"
+fi
+
+"$AGENT_SEND" -L "$SOCKET" -S "tester:source" -s "=$TARGET" -m "agent socket hello" >/tmp/agent-send-named.out
+sleep 0.2
+capture_named | grep -qF "[tester:source ->" || fail "agent-send.sh did not include preamble"
+capture_named | grep -qF "agent socket hello" || fail "agent-send.sh did not deliver to named socket"
+if capture_default | grep -qF "agent socket hello"; then
+  fail "agent-send.sh leaked named-socket message to default tmux server"
+fi
+
+echo "ok - named tmux socket send tools"