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/done.go

@@ -14,11 +14,14 @@ var doneCmd = &cobra.Command{
 	Long: `Mark one or more tasks as completed.
 
 Examples:
-  opal 1 done              # Complete task with display ID 1
-  opal +urgent done        # Complete all urgent tasks
-  opal project:backend done`,
+  opal done 1              # Complete task with display ID 1
+  opal 1 done              # Flexible syntax (same as above)
+  opal done +urgent        # Complete all urgent tasks
+  opal +urgent done        # Flexible syntax (same as above)
+  opal done project:backend`,
 	Run: func(cmd *cobra.Command, args []string) {
-		if err := completeTasks(args); err != nil {
+		parsed := getParsedArgs(cmd)
+		if err := completeTasks(parsed.Filters); err != nil {
 			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 			os.Exit(1)
 		}