Fix three critical UX issues in opal-task

Issue 1: Fix recurrence calculation for overdue tasks
- Use completion date (End) as base for next instance, not original due date
- If task due Monday completed Wednesday, next is Wednesday+7d not Monday+7d
- Fallback to Due date if End is not set
- Update test to verify new behavior

Issue 2: Fix description parsing to work without quotes
- Add parseAddArgs() to extract description from non-modifier words
- Description = all words that don't start with +, -, or contain :
- Enables: opal add buy groceries +shop carrots → 'buy groceries carrots'
- Validate description is required (error if only modifiers)
- Validate recurring tasks require due date

Issue 3: Implement flexible command syntax
- Add preprocessArgs() to parse arguments before Cobra routing
- Detect command position and split filters (left) from modifiers (right)
- Rewrite os.Args so Cobra routes correctly
- Enable both 'opal 2 done' and 'opal done 2' syntax
- Commands without modifiers accept filters on either side
- Commands with modifiers enforce [filters] command [modifiers]
- Add confirmation for modify without filters (modifies all tasks)

All commands updated to use preprocessed ParsedArgs from context.
All tests passing (33 tests).

opal-task/internal/engine/recurrence.go

@@ -76,12 +76,19 @@ func SpawnNextInstance(completedInstance *Task) error {
 	}
 
 	// Calculate next due date
-	var nextDue *time.Time
-	if completedInstance.Due != nil {
-		next := CalculateNextDue(*completedInstance.Due, *template.RecurrenceDuration)
-		nextDue = &next
+	// Use End date (completion time) as base, fallback to Due date if End not set
+	var baseDate time.Time
+	if completedInstance.End != nil {
+		baseDate = *completedInstance.End
+	} else if completedInstance.Due != nil {
+		baseDate = *completedInstance.Due
+	} else {
+		return fmt.Errorf("recurring instance has no due or end date")
 	}
 
+	next := CalculateNextDue(baseDate, *template.RecurrenceDuration)
+	nextDue := &next
+
 	// Check if we're past 'until' date
 	if template.Until != nil && nextDue != nil && nextDue.After(*template.Until) {
 		// Don't spawn, recurrence has expired