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/task.go

@@ -0,0 +1,57 @@
+package engine
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Status byte
+
+const (
+	StatusPending   Status = 'P'
+	StatusCompleted Status = 'C'
+	StatusDeleted   Status = 'D'
+	StatusRecurring Status = 'R'
+)
+
+type Priority int
+
+const (
+	PriorityLow     Priority = 0
+	PriorityDefault Priority = 1
+	PriorityMedium  Priority = 2
+	PriorityHigh    Priority = 3
+)
+
+type Task struct {
+	// Identity
+	UUID uuid.UUID
+	ID   int
+
+	// Core fields
+	Status      Status
+	Description string
+	Project     *string
+	Priority    Priority
+
+	// Timestamps
+	Created   time.Time
+	Modified  time.Time
+	Start     *time.Time
+	End       *time.Time
+	Due       *time.Time
+	Scheduled *time.Time
+	Wait      *time.Time
+	Until     *time.Time
+
+	// Recurrence (parent-child approach)
+	RecurrenceDuration *time.Duration
+	ParentUUID         *uuid.UUID
+
+	// Derived fields (not stored in DB)
+	Tags []string
+}
+
+// timeNow returns current time (allows mocking in tests)
+var timeNow = time.Now