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/cmd/add.go

@@ -15,12 +15,15 @@ var addCmd = &cobra.Command{
 	Long: `Add a new task with optional modifiers.
 
 Examples:
-  opal add "Buy groceries"
-  opal add "Review PR" priority:H project:backend
-  opal add "Team meeting" due:mon recur:1w +meetings`,
-	Args: cobra.MinimumNArgs(1),
+  opal add buy groceries                          # No quotes needed!
+  opal add review PR priority:H project:backend
+  opal add buy groceries +shop carrots            # Tag can be anywhere
+  opal add team meeting due:mon recur:1w +meetings`,
 	Run: func(cmd *cobra.Command, args []string) {
-		if err := addTask(args); err != nil {
+		parsed := getParsedArgs(cmd)
+		// For add, combine filters and modifiers (all are args to parse)
+		allArgs := append(parsed.Filters, parsed.Modifiers...)
+		if err := addTask(allArgs); err != nil {
 			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 			os.Exit(1)
 		}
@@ -28,13 +31,16 @@ Examples:
 }
 
 func addTask(args []string) error {
-	// First arg is description, rest are modifiers
-	description := args[0]
-	modifierArgs := args[1:]
+	// Parse description and modifiers from args
+	// Description = all words that are NOT filters/modifiers
+	// Filters/Modifiers = words with +, -, or containing :
+	description, modifierArgs, err := parseAddArgs(args)
+	if err != nil {
+		return err
+	}
 
 	// Parse modifiers
 	var mod *engine.Modifier
-	var err error
 
 	if len(modifierArgs) > 0 {
 		mod, err = engine.ParseModifier(modifierArgs)
@@ -65,6 +71,33 @@ func addTask(args []string) error {
 	return nil
 }
 
+// parseAddArgs extracts description and modifiers from args
+// Description = all non-filter/modifier words joined with spaces
+// Filters/Modifiers = args with +, -, or containing :
+func parseAddArgs(args []string) (string, []string, error) {
+	var descParts []string
+	var modifiers []string
+
+	for _, arg := range args {
+		isFilterOrModifier := strings.HasPrefix(arg, "+") ||
+			strings.HasPrefix(arg, "-") ||
+			strings.Contains(arg, ":")
+
+		if isFilterOrModifier {
+			modifiers = append(modifiers, arg)
+		} else {
+			descParts = append(descParts, arg)
+		}
+	}
+
+	if len(descParts) == 0 {
+		return "", nil, fmt.Errorf("description is required")
+	}
+
+	description := strings.Join(descParts, " ")
+	return description, modifiers, nil
+}
+
 func addRecurringTask(description string, mod *engine.Modifier) error {
 	// Extract recurrence pattern
 	recurPattern := mod.SetAttributes["recur"]
@@ -72,6 +105,11 @@ func addRecurringTask(description string, mod *engine.Modifier) error {
 		return fmt.Errorf("no recurrence pattern specified")
 	}
 
+	// Validate: recurring tasks must have due date
+	if mod.SetAttributes["due"] == nil {
+		return fmt.Errorf("recurring tasks require a due date (use due:YYYY-MM-DD or due:monday)")
+	}
+
 	duration, err := engine.ParseRecurrencePattern(*recurPattern)
 	if err != nil {
 		return fmt.Errorf("invalid recurrence pattern: %w", err)