feat(config): add projects-sync systemd service and timer

dot_config/systemd/user/projects-sync.timer

@@ -0,0 +1,11 @@
+[Unit]
+Description=Keep ~/Projects in sync with ~/Code
+
+# ponytail: polls every 5 min; ghq clones land two levels below ~/Code, out
+# of reach of a non-recursive .path unit. Switch to inotify if 5 min is slow.
+[Timer]
+OnStartupSec=30s
+OnUnitActiveSec=5min
+
+[Install]
+WantedBy=timers.target