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

AGENT_TODO.md

@@ -49,6 +49,10 @@ summary (newest last).
 
 ## Done (summary, newest last)
 
+- 2026-08-06: Multi-platform alerting — `lib/notify.sh` routes via `NOTIFY_PLATFORM`
+  (`notify.env`, default telegram; sender contract for Matrix/Synapse later),
+  `system.env` shared config for health/backup, dynamic effective values in
+  `--help`, telegram `--markdown` alias.
 - 2026-08-06: Tier 1 — `pos system health` (dashboard + `--send`), `lib/notify.sh`
   (wired into backup + firewall), daily digest timer via postinstall.
 - 2026-08-06: Document Map index + Entertainment section in AGENT_Context (cf36780).