Say each notification in plain words the operator reads at a glance

The desktop popups were walls of plan ids, commits, keys and verb syntax in UTC.
The messenger now shows the condition's headline, explanation and resolved line,
since when in the operator's time zone, and a clearance in one line (hq ADR 0253).

modules/messenger/cmd/messenger/condition.go

@@ -17,6 +17,8 @@ package main
 //   - `severity` is `urgent` or `warning`. Anything else is not guessed: the event is unreadable.
 //   - `silenced` is absent, null, or an object {until, by, why}; `until` in the past is not silenced.
 //   - Times are RFC 3339.
+//   - `headline`, `explanation` and `resolved` are the plain words the operator reads (novox/hq ADR
+//     0253): a condition from a controller older than them has none, and is said from its summary.
 //   - `key` is `<scope>.<id>.<kind>`. When it is absent it is made from subject and kind; when
 //     neither gives one the event is unreadable.
 //   - A cleared event carries the condition as last held, and may add `cleared` (its time).
@@ -60,6 +62,9 @@ type Condition struct {
 	Machine      string
 	Severity     string
 	Summary      string
+	Headline     string
+	Explanation  string
+	Resolved     string
 	Source       string
 	Resolver     string
 	Raised       time.Time
@@ -164,6 +169,9 @@ func DecodeCondition(event string, body []byte) (Condition, error) {
 	c.Kind = str("kind")
 	c.Severity = str("severity")
 	c.Summary = str("summary")
+	c.Headline = str("headline")
+	c.Explanation = str("explanation")
+	c.Resolved = str("resolved")
 	c.Resolver = str("resolver")
 	c.Raised = when("raised")
 	c.LastObserved = when("last-observed", "last_observed", "lastObserved")