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.

config/event.env

@@ -0,0 +1,21 @@
+# eventer rules — evaluated by 'pos system event-trigger run' (systemd timer).
+# Copied to ~/.config/linux_post_install/event.env on install (no clobber).
+# One independent rule per line. Grammar:
+#
+#   "<message>" if <check-command> <op> <threshold>
+#   <check-command> <op> <threshold>          (no message → auto-composed)
+#
+# The check command is run; its first numeric output is compared against the
+# threshold (float-safe, unit suffix on the threshold is fine: 60c, 80%, 10g).
+# Operators:  >  <  >=  <=  ==  !=
+#
+# Rules alert once on false→true (plus one recovery message when the
+# condition clears) via lib/notify.sh — Telegram by default.
+#
+# Examples:
+#   "CPU too hot" if sensors -u | grep -m1 temp1_input | awk '{print $2}' > 60c
+#   "Disk nearly full" if df -P / | awk 'NR==2{print $5+0}' > 80%
+#   "Load high" if uptime | sed 's/.*load average: //; s/,.*//' >= 4
+#
+# Manage rules interactively:   pos system event-trigger config
+# Schedule the timer:           pos system event-trigger enable 5m