fix: use Modifier.Set() to maintain AttributeOrder invariant in API handlers

API handlers were populating SetAttributes directly without appending to
AttributeOrder. Since Apply() only iterates AttributeOrder, all updates
via PUT/POST were silently dropped — causing edits to revert and tasks
to disappear on reload.

Adds Modifier.Set() helper, safety net in Apply()/ApplyToNew(), adds
description and status to applyNonDateAttribute, and fixes the
recurrence->recur key mismatch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/keys.go

@@ -4,14 +4,15 @@ package engine
 // Used by parseAddArgs (cmd/add.go), ParseFilter, and ParseModifier
 // to distinguish modifiers from description text.
 var ValidAttributeKeys = map[string]bool{
-	"due":       true,
-	"priority":  true,
-	"project":   true,
-	"recur":     true,
-	"status":    true,
-	"wait":      true,
-	"scheduled": true,
-	"until":     true,
+	"description": true,
+	"due":         true,
+	"priority":    true,
+	"project":     true,
+	"recur":       true,
+	"status":      true,
+	"wait":        true,
+	"scheduled":   true,
+	"until":       true,
 }
 
 // DateKeys is the subset of ValidAttributeKeys that hold date values.