feat: add systemd service config and install script

config/systemd/user/claude-sessions-save.service: source of truth
for the shutdown service, version-controlled alongside the scripts
it calls.

scripts/install-systemd.sh: idempotent install script — copies the
service file to ~/.config/systemd/user/, reloads the daemon, and
enables + starts the service. Run on any new machine to wire up
graceful Claude session shutdown.

scripts/install-systemd.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# install-systemd.sh — Install claude-sessions systemd user service.
+#
+# Copies the service file from this repo into ~/.config/systemd/user/,
+# reloads the daemon, and enables + starts the service.
+#
+# Safe to re-run (idempotent).
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_DIR="$(dirname "$SCRIPT_DIR")"
+SRC="$REPO_DIR/config/systemd/user/claude-sessions-save.service"
+DST_DIR="$HOME/.config/systemd/user"
+DST="$DST_DIR/claude-sessions-save.service"
+
+GREEN='\033[0;32m'; YELLOW='\033[1;33m'; NC='\033[0m'
+ok()   { echo -e "${GREEN}OK${NC}    $*"; }
+skip() { echo -e "${YELLOW}SKIP${NC}  $*"; }
+
+[[ -f "$SRC" ]] || { echo "ERROR: source not found: $SRC" >&2; exit 1; }
+
+mkdir -p "$DST_DIR"
+
+if [[ -f "$DST" ]] && diff -q "$SRC" "$DST" &>/dev/null; then
+    skip "service file already up to date"
+else
+    cp "$SRC" "$DST"
+    ok "installed $DST"
+fi
+
+systemctl --user daemon-reload
+ok "daemon reloaded"
+
+if systemctl --user is-enabled --quiet claude-sessions-save.service 2>/dev/null; then
+    skip "already enabled"
+else
+    systemctl --user enable claude-sessions-save.service
+    ok "enabled claude-sessions-save.service"
+fi
+
+if systemctl --user is-active --quiet claude-sessions-save.service 2>/dev/null; then
+    skip "already active"
+else
+    systemctl --user start claude-sessions-save.service
+    ok "started claude-sessions-save.service"
+fi
+
+echo ""
+echo "claude-sessions-save.service is enabled and active."
+echo "It will gracefully exit Claude tmux sessions at shutdown."