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/list.go

@@ -1,3 +1,56 @@
 package cmd
 
-// TODO: Implement list command
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var listCmd = &cobra.Command{
+	Use:   "list [filter...]",
+	Short: "List tasks",
+	Long: `List tasks matching the filter criteria.
+
+Examples:
+  opal list                    # List all pending tasks
+  opal list +home              # List tasks with +home tag
+  opal list project:backend    # List backend project tasks
+  opal list priority:H         # List high priority tasks`,
+	Run: func(cmd *cobra.Command, args []string) {
+		if err := listTasks(args); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func listTasks(args []string) error {
+	// Parse filter
+	var filter *engine.Filter
+	var err error
+
+	if len(args) == 0 {
+		filter = engine.DefaultFilter()
+	} else {
+		filter, err = engine.ParseFilter(args)
+		if err != nil {
+			return fmt.Errorf("failed to parse filter: %w", err)
+		}
+	}
+
+	// Build working set
+	ws, err := engine.BuildWorkingSet(filter)
+	if err != nil {
+		return fmt.Errorf("failed to build working set: %w", err)
+	}
+
+	// Get tasks
+	tasks := ws.GetTasks()
+
+	// Display
+	fmt.Println(engine.FormatTaskList(tasks, ws))
+
+	return nil
+}