feat: add annotations, undo system, and schema updates

Add annotations as JSON column on tasks table with Annotate/Denotate
methods and CLI commands. Add undo system backed by change_log with
lightweight undo_stack table (capped at 10 entries). All mutating CLI
commands (add, done, delete, modify, start, stop) now record undo
entries. Undo restores prior task state from change_log data.

Schema changes (in v1 migration):
- annotations TEXT column on tasks
- undo_stack table
- annotations field in change_log triggers

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

opal-task/cmd/edit.go

@@ -198,6 +198,17 @@ func generateEditableContent(task *engine.Task) string {
 		sb.WriteString("tags: \n")
 	}
 
+	// Annotations
+	sb.WriteString("\n# Annotations (add/remove/modify lines below)\n")
+	if len(task.Annotations) > 0 {
+		for i, ann := range task.Annotations {
+			ts := time.Unix(ann.Timestamp, 0).Format("2006-01-02 15:04")
+			sb.WriteString(fmt.Sprintf("annotation.%d: %s | %s\n", i+1, ts, ann.Text))
+		}
+	}
+	sb.WriteString("# To add: annotation.N: YYYY-MM-DD HH:MM | text\n")
+	sb.WriteString("# To remove: delete the line\n")
+
 	return sb.String()
 }
 
@@ -362,6 +373,32 @@ func applyNonStatusFields(task *engine.Task, fields map[string]string) error {
 		}
 	}
 
+	// Annotations - rebuild from annotation.N fields
+	var newAnnotations []engine.Annotation
+	for key, value := range fields {
+		if strings.HasPrefix(key, "annotation.") && value != "" {
+			parts := strings.SplitN(value, " | ", 2)
+			if len(parts) == 2 {
+				ts, err := time.Parse("2006-01-02 15:04", strings.TrimSpace(parts[0]))
+				if err != nil {
+					// If we can't parse the timestamp, use current time
+					ts = time.Now()
+				}
+				newAnnotations = append(newAnnotations, engine.Annotation{
+					Timestamp: ts.Unix(),
+					Text:      strings.TrimSpace(parts[1]),
+				})
+			} else {
+				// No timestamp separator, treat entire value as text
+				newAnnotations = append(newAnnotations, engine.Annotation{
+					Timestamp: time.Now().Unix(),
+					Text:      strings.TrimSpace(value),
+				})
+			}
+		}
+	}
+	task.Annotations = newAnnotations
+
 	// Tags - replace all tags
 	if tagsStr, ok := fields["tags"]; ok {
 		// Remove all existing tags