morning: an item can be optional, so a skipped stretch is not a skipped pill (V-473)

Item carried only Key, FactKey and Label, so every checklist entry was
implicitly required and behaviour 1 of #280 could not hold at all. It was not
thin config — there was no field to set.

Item.Optional, `"optional": true` in the routine config, default false, so a
routine written before today behaves exactly as it did. Due now fires on a
missing required item and not on an optional one, and the optional stragglers
still travel in Missing so the one message per day per routine can name them
after the required ones, in softer words.

Evidence, the window and the day plan treat both kinds alike. A missing
optional item is still missing — it just does not earn a nudge, because a
checklist where everything is mandatory is one he learns to ignore.

internal/config/config.go

@@ -601,6 +601,9 @@ type MorningRoutineItemConfig struct {
 	Key     string `json:"key"`
 	FactKey string `json:"fact_key"`
 	Label   string `json:"label"`
+	// Optional — this one being skipped does not earn a nudge. Default false,
+	// so a routine written before 04-08-2026 keeps behaving as it did.
+	Optional bool `json:"optional,omitempty"`
 }
 
 // QuietHoursConfig — a recurring daily quiet-window. Times are local to the
@@ -1734,7 +1737,7 @@ func morningRoutinesFromConfig(mc []MorningRoutineConfig) []morning.Routine {
 	for i, r := range mc {
 		items := make([]morning.Item, len(r.Items))
 		for j, it := range r.Items {
-			items[j] = morning.Item{Key: it.Key, FactKey: it.FactKey, Label: it.Label}
+			items[j] = morning.Item{Key: it.Key, FactKey: it.FactKey, Label: it.Label, Optional: it.Optional}
 		}
 		weekdays := make([]time.Weekday, len(r.Weekdays))
 		for j, w := range r.Weekdays {