feat: Phase 2 - Sync infrastructure

- Created sync client for communicating with API server
- Implemented conflict resolution strategies (last-write-wins, server-wins, client-wins)
- Added offline change queue for queuing changes when server is unreachable
- Implemented merge logic for local and remote task lists
- Added conflict logging to sync_conflicts.log
- Created bidirectional sync with pull/push operations
- Extended Config struct with sync settings (URL, API key, client ID, strategy, offline queue)
- Added SyncResult display with user-friendly output
- Sync handlers already implemented in Phase 1 (GetChanges, PushChanges)

opal-task/internal/sync/queue.go

@@ -0,0 +1,118 @@
+package sync
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/google/uuid"
+)
+
+// QueuedChange represents a change waiting to be synced
+type QueuedChange struct {
+	ID        string          `json:"id"`
+	Timestamp int64           `json:"timestamp"`
+	TaskUUID  uuid.UUID       `json:"task_uuid"`
+	Type      string          `json:"type"` // create, update, delete
+	Data      json.RawMessage `json:"data"`
+}
+
+// Queue manages offline changes
+type Queue struct {
+	filepath string
+	changes  []QueuedChange
+}
+
+// NewQueue creates a new queue instance
+func NewQueue() (*Queue, error) {
+	configDir, err := engine.GetConfigDir()
+	if err != nil {
+		return nil, err
+	}
+
+	queuePath := filepath.Join(configDir, "sync_queue.json")
+	q := &Queue{filepath: queuePath}
+
+	if err := q.load(); err != nil {
+		// If file doesn't exist, start with empty queue
+		if !os.IsNotExist(err) {
+			return nil, err
+		}
+		q.changes = []QueuedChange{}
+	}
+
+	return q, nil
+}
+
+// Add adds a change to the queue
+func (q *Queue) Add(change QueuedChange) error {
+	q.changes = append(q.changes, change)
+	return q.save()
+}
+
+// GetPending returns all pending changes
+func (q *Queue) GetPending() []QueuedChange {
+	return q.changes
+}
+
+// Clear removes all changes from the queue
+func (q *Queue) Clear() error {
+	q.changes = []QueuedChange{}
+	return q.save()
+}
+
+// Size returns the number of pending changes
+func (q *Queue) Size() int {
+	return len(q.changes)
+}
+
+// load reads the queue from disk
+func (q *Queue) load() error {
+	data, err := os.ReadFile(q.filepath)
+	if err != nil {
+		return err
+	}
+
+	return json.Unmarshal(data, &q.changes)
+}
+
+// save writes the queue to disk
+func (q *Queue) save() error {
+	data, err := json.MarshalIndent(q.changes, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(q.filepath, data, 0644)
+}
+
+// QueueLocalChanges adds local task changes to the queue
+func QueueLocalChanges(tasks []*engine.Task) error {
+	queue, err := NewQueue()
+	if err != nil {
+		return fmt.Errorf("failed to create queue: %w", err)
+	}
+
+	for _, task := range tasks {
+		taskData, err := json.Marshal(task)
+		if err != nil {
+			continue
+		}
+
+		change := QueuedChange{
+			ID:        uuid.New().String(),
+			Timestamp: task.Modified.Unix(),
+			TaskUUID:  task.UUID,
+			Type:      "update",
+			Data:      taskData,
+		}
+
+		if err := queue.Add(change); err != nil {
+			return fmt.Errorf("failed to queue change: %w", err)
+		}
+	}
+
+	return nil
+}