deploy, docs: turn service_down back on (V-444)

It was disabled because it could not say which service. It can now.

deploy/mavend.json

@@ -8,12 +8,12 @@
   "//disabled_rules": [
     "Nudge rules that are not wired at all. Names come from loop.DefaultRules:",
     "water, meal, break, service_down, netdata_critical.",
-    "service_down is off because it cannot say WHICH service — mavpoll folds the",
-    "whole kuma gauge into one boolean, so the nudge is always the generic 'a",
-    "service on homesrv is down'. Nothing to act on, every fifteen minutes.",
-    "Turn it back on once Vikunja #444 lands a fact per monitor."
+    "service_down is back on: mavpoll now writes one fact per kuma monitor",
+    "(service_down:<name>), so the nudge names the service and pausing a monitor",
+    "in kuma silences that monitor. It is also edge-triggered, so a service that",
+    "stays down is one nudge, not one every fifteen minutes."
   ],
-  "disabled_rules": ["service_down"],
+  "disabled_rules": [],
 
   "phraser": {
     "model_path": "/opt/maven/models/llm/qwen3/Qwen3-1.7B-UD-Q4_K_XL.gguf",

docs/design.md

@@ -389,6 +389,29 @@ lives in `source`; rules trust provenance.
   `source=poll:healthcheck`. A compromised poller must not be able to forge a
   trigger.
 
+#### A fact per monitor, not an aggregate
+
+mavpoll writes one fact per kuma monitor, keyed `service_down:<monitor name>`.
+It used to fold the whole gauge into a single boolean, and the nudge could then
+only say that something on homesrv was down. That is not something he can act
+on, so the rule shipped disabled.
+
+Three things follow from the split:
+
+- The key set is no longer known at wiring time. A rule declares
+  `WantPrefixes` and the gatherer resolves the family per tick, which is the
+  only prefix read in the loop.
+- Pausing a monitor in kuma silences that monitor. Under the aggregate it
+  silenced nothing, because some other monitor kept the boolean at "down".
+- A monitor deleted in kuma would keep its last fact reading "down" forever, so
+  mavpoll marks a vanished monitor "unknown". No rule fires on "unknown".
+
+The rule is also edge-triggered: it fires on a transition it has not already
+nudged about (`State.NudgedSince`). A polled fact is written only when the
+value changes, but the predicate reads the current value, so without the edge
+check a service that stays down qualifies on every tick and cooldown is the
+only brake.
+
 ### Presence — concrete scoring
 
 **Combiner — noisy-OR, not weighted sum.** These are independent-ish positive

internal/config/config.go

@@ -149,10 +149,11 @@ type Config struct {
 	//
 	// Rules are code, not config (see loop.DefaultRules), and that stays true:
 	// this only subtracts. It exists because a rule can be right in principle
-	// and useless in practice — kuma's service_down cannot name the service it
-	// is nudging about (Vikunja #444), so being told "a service on homesrv is
-	// down" every fifteen minutes is noise with no action attached. Turning it
-	// off beats learning to ignore her.
+	// and useless in practice. service_down was the case that forced it: it
+	// could not name the service it was nudging about, so being told "a service
+	// on homesrv is down" every fifteen minutes was noise with no action
+	// attached. That is fixed — one fact per kuma monitor — and the rule ships
+	// enabled again. The escape hatch stays.
 	//
 	// A disabled rule is never gathered for, never evaluated, and never
 	// delivered on any channel. Unknown names are ignored, so removing a rule