feat: pos system event-trigger — state-based threshold monitors (eventer)

Each line of event.env is an independent rule: ["msg" if ] <check> <op> <thr>.
Check runs on every pass; first numeric output compared float-safe; op is the
rightmost 'op threshold' pair so checks with their own >/< parse fine. Alerts
once on false->true + one recovery message on true->false (no repeats while
the condition holds); per-rule state keyed by rule-line hash in
~/.local/share/linux_post_install/eventer/state/.

Subcommands: run (timer entrypoint), config (interactive add/remove/edit with
check-validation), list (rules + live values), enable [interval] (systemd
user timer pos-event-trigger.timer; 5m..weekly or OnCalendar; graceful without
a user manager, loginctl enable-linger attempt), disable, status. --dry-run
honors the DEV.md convention. Alerts via lib/notify.sh (Telegram default).

New: bin/pos-system-event-trigger, lib/eventer-lib.sh, config/event.env
template (no-clobber via postinstall), install.sh lib install, INTERACTIVE_CMDS
entry. Docs: POS.md system row, HOWTO.md index, howto/event-trigger.md.
make gen && make check green; functional tests cover trigger/recovery/no-repeat,
float+unit parsing, editor add/remove/edit + validation + dry-run, timer
enable/disable/status, dispatcher routing.

completions/pos.bash

@@ -11,6 +11,7 @@ _pos_flags[media-mp3]="--output --no-playlist --cookies --by-artist --dry-run"
 _pos_flags[media-mp4]="--format --best --worst --output --no-playlist --cookies --dry-run"
 _pos_flags[network-hotspot]="--foreground"
 _pos_flags[system-backup]="--service"
+_pos_flags[system-event-trigger]="--dry-run"
 _pos_flags[tree]="--depth"
 _pos_flags[usb-server]="--ls --ls-shared --share --unshare --auto-share --callback --close-callback --auto-connect --disconnect --nickname --timeout --port --info --version"
 # GEN:END posflags
@@ -20,6 +21,7 @@ _pos_subcmds[ai-gemini]="ask chat models sessions"
 _pos_subcmds[communication-telegram-sender]="send test"
 _pos_subcmds[docker-compose]="ls installed up down restart logs update config"
 _pos_subcmds[docker-vbox]="create enter stop start rm ls"
+_pos_subcmds[system-event-trigger]="run config list enable disable status"
 # GEN:END possubcmds
 # GEN:START posconfigscopes
 declare -a _pos_config_scopes=(ai compose entertainment notify system telegram)