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/done.go

@@ -1,3 +1,91 @@
 package cmd
 
-// TODO: Implement done command
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var doneCmd = &cobra.Command{
+	Use:   "done [filter...]",
+	Short: "Mark tasks as completed",
+	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`,
+	Run: func(cmd *cobra.Command, args []string) {
+		if err := completeTasks(args); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func completeTasks(args []string) error {
+	// Parse filter
+	filter, err := engine.ParseFilter(args)
+	if err != nil {
+		return fmt.Errorf("failed to parse filter: %w", err)
+	}
+
+	// Load working set to resolve IDs
+	ws, err := engine.LoadWorkingSet()
+	if err != nil {
+		return fmt.Errorf("failed to load working set: %w", err)
+	}
+
+	// Get matching tasks
+	var tasks []*engine.Task
+
+	if len(filter.IDs) > 0 {
+		// Resolve display IDs
+		for _, id := range filter.IDs {
+			task, err := ws.GetTaskByDisplayID(id)
+			if err != nil {
+				return err
+			}
+			tasks = append(tasks, task)
+		}
+	} else {
+		// Use filter to get tasks
+		matched, err := engine.GetTasks(filter)
+		if err != nil {
+			return fmt.Errorf("failed to get tasks: %w", err)
+		}
+		tasks = matched
+	}
+
+	if len(tasks) == 0 {
+		fmt.Println("No tasks matched.")
+		return nil
+	}
+
+	// Confirm if multiple tasks
+	if len(tasks) > 1 {
+		fmt.Printf("About to complete %d tasks. Proceed? (y/N): ", len(tasks))
+		var confirm string
+		fmt.Scanln(&confirm)
+		if confirm != "y" && confirm != "Y" {
+			fmt.Println("Cancelled.")
+			return nil
+		}
+	}
+
+	// Complete tasks
+	completed := 0
+	for _, task := range tasks {
+		if err := task.Complete(); err != nil {
+			fmt.Fprintf(os.Stderr, "Warning: failed to complete task %s: %v\n", task.UUID, err)
+		} else {
+			completed++
+		}
+	}
+
+	fmt.Printf("Completed %d task(s).\n", completed)
+
+	return nil
+}