Implement opal-task Phase 1: Database foundation

- Add SQLite database with schema for tasks, tags, and working_set
- Implement config management with Viper (opal.yml)
- Create Task struct with proper types (*time.Time, Priority int)
- Add database migration system
- Implement recurrence pattern parsing (1d, 1w, 1m, 1y)
- Setup project structure with cmd/ and internal/engine/
- Add dependencies: sqlite3, uuid, cobra, viper, color

opal-task/internal/engine/recurrence.go

@@ -0,0 +1,69 @@
+package engine
+
+import (
+	"fmt"
+	"strconv"
+	"time"
+)
+
+// ParseRecurrencePattern converts "1w", "2d", "1m" to time.Duration
+func ParseRecurrencePattern(pattern string) (time.Duration, error) {
+	if len(pattern) < 2 {
+		return 0, fmt.Errorf("invalid recurrence pattern: %s", pattern)
+	}
+
+	numStr := pattern[:len(pattern)-1]
+	unit := pattern[len(pattern)-1]
+
+	num, err := strconv.Atoi(numStr)
+	if err != nil {
+		return 0, fmt.Errorf("invalid number in pattern: %s", pattern)
+	}
+
+	switch unit {
+	case 'd':
+		return time.Duration(num) * 24 * time.Hour, nil
+	case 'w':
+		return time.Duration(num) * 7 * 24 * time.Hour, nil
+	case 'm':
+		// Approximate: 30 days
+		return time.Duration(num) * 30 * 24 * time.Hour, nil
+	case 'y':
+		// Approximate: 365 days
+		return time.Duration(num) * 365 * 24 * time.Hour, nil
+	default:
+		return 0, fmt.Errorf("unknown unit: %c (use d/w/m/y)", unit)
+	}
+}
+
+// FormatRecurrenceDuration converts time.Duration back to "1w", "2d" format
+func FormatRecurrenceDuration(d time.Duration) string {
+	days := int(d.Hours() / 24)
+
+	if days%365 == 0 && days/365 > 0 {
+		return fmt.Sprintf("%dy", days/365)
+	}
+	if days%30 == 0 && days/30 > 0 {
+		return fmt.Sprintf("%dm", days/30)
+	}
+	if days%7 == 0 && days/7 > 0 {
+		return fmt.Sprintf("%dw", days/7)
+	}
+	return fmt.Sprintf("%dd", days)
+}
+
+// CalculateNextDue calculates next due date based on current and recurrence
+func CalculateNextDue(currentDue time.Time, recurrence time.Duration) time.Time {
+	return currentDue.Add(recurrence)
+}
+
+// SpawnNextInstance creates a new task instance from completed recurring task
+// This will be implemented after we have the CRUD operations
+func SpawnNextInstance(completedInstance *Task) error {
+	if completedInstance.ParentUUID == nil {
+		return fmt.Errorf("task is not a recurring instance")
+	}
+
+	// TODO: Implement after GetTask is available
+	return fmt.Errorf("not implemented yet")
+}