Implement opal-task Phases 6-8: Complete CLI Implementation

Phase 6: Display and Basic Commands
- Add display.go with colored formatting for tasks, projects, tags
- Implement cmd/root.go with Cobra command structure
- Implement cmd/list.go for listing and filtering tasks
- Implement cmd/add.go with support for regular and recurring tasks
- Implement cmd/done.go with bulk completion and confirmation

Phase 7: Advanced Commands
- Implement cmd/modify.go for updating task attributes
- Implement cmd/delete.go with soft delete confirmation
- Implement cmd/start.go and cmd/stop.go for task timing
- Implement cmd/count.go for counting filtered tasks
- Implement cmd/projects.go and cmd/tags.go for aggregation

Phase 8: Integration and Polish
- Update main.go to use CLI commands
- Add colored output with fatih/color
- Format task lists with proper alignment
- Highlight overdue tasks in red, upcoming in yellow
- Test end-to-end workflow: add, list, done, recurring tasks
- Verify recurrence spawning works correctly

All CLI commands functional and tested!

opal-task/cmd/stop.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var stopCmd = &cobra.Command{
+	Use:   "stop [filter...]",
+	Short: "Stop a task (clear start time)",
+	Run: func(cmd *cobra.Command, args []string) {
+		if err := stopTasks(args); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func stopTasks(args []string) error {
+	filter, err := engine.ParseFilter(args)
+	if err != nil {
+		return err
+	}
+
+	ws, _ := engine.LoadWorkingSet()
+	var tasks []*engine.Task
+
+	if len(filter.IDs) > 0 && ws != nil {
+		for _, id := range filter.IDs {
+			task, err := ws.GetTaskByDisplayID(id)
+			if err == nil {
+				tasks = append(tasks, task)
+			}
+		}
+	} else {
+		tasks, err = engine.GetTasks(filter)
+		if err != nil {
+			return err
+		}
+	}
+
+	for _, task := range tasks {
+		task.StopTask()
+		fmt.Printf("Stopped task: %s\n", task.Description)
+	}
+
+	return nil
+}