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.

DOC/howto/event-trigger.md

@@ -0,0 +1,75 @@
+# How-To: `pos system event-trigger`
+
+State-based threshold monitors: each line of `event.env` is an independent rule;
+when a rule's check crosses its threshold you get one alert (plus one recovery
+message when it clears). Alerts go through `lib/notify.sh` — Telegram by
+default, `NOTIFY_PLATFORM` for more.
+
+```bash
+pos system event-trigger config          # interactive rule editor
+pos system event-trigger list            # rules + live check values
+pos system event-trigger enable 5m       # evaluate every 5 minutes via systemd
+pos system event-trigger status          # timer + rule count
+pos system event-trigger disable         # stop monitoring
+pos system event-trigger run --dry-run   # preview what would fire
+```
+
+---
+
+## Rule format
+
+One rule per line in `~/.config/linux_post_install/event.env` (chmod 600):
+
+```
+["<message>" if ] <check-command> <op> <threshold>
+```
+
+| Part | Meaning |
+|------|---------|
+| `"<message>" if` | Optional custom alert text (quote-stripped); without it the message is auto-composed |
+| `<check-command>` | Any shell command; its **first numeric output** is the value (pipes/args fine) |
+| `<op>` | `>` `<` `>=` `<=` `==` `!=` |
+| `<threshold>` | Number with optional unit suffix — `60c`, `80%`, `10g` all work |
+
+The operator is detected as the rightmost `op threshold` pair in the line, so
+check commands containing their own `>`/`<` (redirection, awk) don't confuse it.
+
+Examples:
+
+```
+# event.env
+"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
+```
+
+Behavior:
+
+- The check runs on every pass. Non-numeric/empty output, or an unparseable
+  line → the rule is skipped with a warning (other rules still run).
+- Alerts fire **once** when the condition turns true, and once more when it
+  recovers — a hot CPU for two hours is one message, not twenty.
+- State is tracked per rule in `~/.local/share/linux_post_install/eventer/state/`
+  (keyed by a hash of the rule line — editing a rule resets its state).
+
+## Scheduling
+
+`enable [interval]` installs a systemd **user timer** (`pos-event-trigger.timer`
++ oneshot `.service` that runs `pos system event-trigger run`). Intervals:
+`5m 10m 15m 30m 45m hourly 2h 6h 12h daily weekly`, or a raw `OnCalendar=…`.
+Requires a reachable user systemd manager; run
+`sudo loginctl enable-linger $USER` once so timers fire without login (the tool
+tries this and warns if it can't).
+
+## Alerting
+
+`run` sends via `lib/notify.sh notify_send`, which delivers to every platform in
+`NOTIFY_PLATFORM` (default `telegram`; comma-separated = fan out). Adding a
+Matrix/Synapse sender later needs no changes here — see DOC/DEV.md → Alerting.
+
+---
+
+## Related
+
+- Reference: [DOC/POS.md → system](../POS.md#system)
+- Notifications: [communication](howto/communication.md) / `lib/notify.sh`