docs: plan durable tmux fleet install (#557)

packages/mosaic/framework/tools/fleet/test-start-agent-session.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$(dirname -- "$0")" && pwd)
+START="$SCRIPT_DIR/start-agent-session.sh"
+SOCKET="mosaic-agent-test-$RANDOM-$$"
+AGENT="agent-$RANDOM"
+WORKDIR=$(mktemp -d)
+trap 'tmux -L "$SOCKET" kill-server >/dev/null 2>&1 || true; rm -rf "$WORKDIR"' EXIT
+
+fail() {
+  echo "FAIL: $*" >&2
+  exit 1
+}
+
+MOSAIC_TMUX_SOCKET="$SOCKET" \
+MOSAIC_AGENT_WORKDIR="$WORKDIR" \
+MOSAIC_AGENT_COMMAND='bash --noprofile --norc -i' \
+  "$START" "$AGENT"
+
+tmux -L "$SOCKET" has-session -t "=$AGENT:0.0" || fail "agent session was not created"
+actual_dir=$(tmux -L "$SOCKET" display-message -p -t "=$AGENT:0.0" '#{pane_current_path}')
+[ "$actual_dir" = "$WORKDIR" ] || fail "agent workdir mismatch: $actual_dir"
+
+MOSAIC_TMUX_SOCKET="$SOCKET" \
+MOSAIC_AGENT_WORKDIR="$WORKDIR" \
+MOSAIC_AGENT_COMMAND='bash --noprofile --norc -i' \
+  "$START" "$AGENT" >/tmp/mosaic-start-agent-idempotent.out
+
+grep -qF 'already running' /tmp/mosaic-start-agent-idempotent.out || fail "duplicate start was not idempotent"
+
+echo "ok - start-agent-session"