feat(parser): add ToRemind method to Task for Remind-compatible string output

Implements Task.ToRemind(), which generates a Remind-formatted string
including due date, metadata, projects, contexts, completion status, and
priority.

internal/parser/parser.go

@@ -25,6 +25,75 @@ type Task struct {
 	Metadata       map[string]string `json:"metadata"`
 }
 
+func (t Task) ToRemind() string {
+	var sb strings.Builder
+	sb.WriteString("REM TODO ")
+	if t.DueDate == nil {
+		return ""
+	}
+	sb.WriteString(t.DueDate.Format("2006-01-02"))
+
+	sb.WriteString(" ++5 INFO \"Calendar: TODO.TX\" ")
+
+	if len(t.Metadata) > 0 {
+		for k, v := range t.Metadata {
+			if k == "due" {
+				continue
+			}
+			// k must have the first letter uppercase for Remind
+			k = strings.ToUpper(k[:1]) + k[1:]
+			sb.WriteString(fmt.Sprintf("INFO \"%s: %s\" ", k, v))
+		}
+	}
+
+	if len(t.Projects) > 0 {
+		for _, p := range t.Projects {
+			sb.WriteString(fmt.Sprintf("INFO \"List: %s\" ", p))
+		}
+	}
+
+	if len(t.Contexts) > 0 {
+		for _, c := range t.Contexts {
+			sb.WriteString(fmt.Sprintf("INFO \"Tag: %s\" ", c))
+		}
+	}
+
+	if t.Completed {
+		sb.WriteString(fmt.Sprintf("COMPLETE-THROUGH %s ", t.DueDate.Format("2006-01-02")))
+	}
+
+	if t.Priority != nil {
+		sb.WriteString(fmt.Sprintf("PRIORITY %d ", t.PriorityAsRemind()))
+	}
+
+	sb.WriteString("MSG")
+
+	if len(t.Projects) > 0 {
+		sb.WriteString(" %<List>:")
+	}
+
+	sb.WriteString(fmt.Sprintf(" %s", t.Description))
+
+	if t.Completed {
+		sb.WriteString("%:")
+	} else {
+		sb.WriteString(" (%b)")
+	}
+
+	if len(t.Metadata) > 0 {
+		for k := range t.Metadata {
+			if k == "due" {
+				continue
+			}
+			// uppercase first letter for Remind
+			k = strings.ToUpper(k[:1]) + k[1:]
+			sb.WriteString(fmt.Sprintf("%%_%s: %%<%s>", k, k))
+		}
+	}
+
+	return sb.String()
+}
+
 // PriorityAsRemind returns an integer representing the priority for sorting.
 // A = 9999 (highest), Z = 0 (lowest), absent = 5000.
 func (t Task) PriorityAsRemind() int {