feat: multi-platform alerting + shared system.env config with dynamic help values

- lib/notify.sh: route notify_send to every platform in NOTIFY_PLATFORM
  (notify.env, default telegram; comma-separated = send to all). New
  platforms need only a bin/pos-communication-<p> sender implementing
  'send <value> [--markdown]' (Matrix/Synapse ready)
- pos-communication-telegram: add --markdown as alias for --parse-mode
  markdown to match the sender contract
- lib/common.sh: load_system_env() — shared ~/.config/linux_post_install/
  system.env for pos-system-* tools (env exported > file > default)
- pos-system-health/backup: load system.env and show effective dynamic
  values (NOTIFY_PLATFORM, HEALTH_BACKUP_MAX_AGE_DAYS, BACKUP_SERVICE_ROOTS)
  in --help
- config/system.env + config/notify.env templates copied by postinstall
- systemd/pos-health.service: EnvironmentFile for both configs so the
  daily digest honors them

systemd/pos-health.service

@@ -1,9 +1,11 @@
 [Unit]
-Description=POS Health digest (daily report via Telegram)
+Description=POS Health digest (daily report via configured notify platforms)
 After=network-online.target
 Wants=network-online.target
 
 [Service]
 Type=oneshot
 User=__POS_USER__
+EnvironmentFile=-%h/.config/linux_post_install/system.env
+EnvironmentFile=-%h/.config/linux_post_install/notify.env
 ExecStart=/usr/local/bin/pos system health --send --markdown