feat: improve CLI output with relative dates, rich feedback, and recurring task info

Add relative date formatting (today, tomorrow, in 3d, etc.) for list and
detail views. Add structured feedback helpers for add/complete/delete
operations showing display IDs and parsed modifiers. Change Complete() to
return spawned recurring instance so callers can display recurrence info.
Add AppendTask to working set for immediate display ID assignment.

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

opal-task/internal/engine/task_test.go

@@ -165,7 +165,7 @@ func TestTaskComplete(t *testing.T) {
 		t.Fatalf("Failed to create task: %v", err)
 	}
 
-	if err := task.Complete(); err != nil {
+	if _, err := task.Complete(); err != nil {
 		t.Fatalf("Failed to complete task: %v", err)
 	}