fix: break sync feedback loop, respect timestamps, surface errors

- Add migration v2: source column on change_log to distinguish local
  vs sync-originated entries, preventing the echo loop where synced
  tasks get re-pushed as local changes
- PushChanges handler now skips save when server version is newer
- Client PushChanges/pushQueuedChanges collect and report marshal errors
  instead of silently dropping them
- De-duplicate getLocalChanges/getLastSyncTime into exported sync
  package functions
- Fix logConflict winner detection via pointer identity instead of
  fragile UUID+timestamp comparison
- Fix sync down to actually parse, save, and tag-sync pulled changes

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

opal-task/internal/sync/strategy.go

@@ -57,7 +57,11 @@ func MergeTasks(local, remote []*engine.Task, strategy ConflictResolution) ([]*e
 		if DetectConflict(task, remoteTask) {
 			conflicts++
 			winner := resolveConflict(task, remoteTask, strategy)
-			logConflict(task, remoteTask, winner)
+			winnerLabel := "local"
+			if winner == remoteTask {
+				winnerLabel = "remote"
+			}
+			logConflict(task, remoteTask, winnerLabel)
 			result = append(result, winner)
 		} else {
 			// No conflict - use either (same content)
@@ -110,17 +114,12 @@ func resolveConflict(local, remote *engine.Task, strategy ConflictResolution) *e
 }
 
 // logConflict writes conflict information to log file
-func logConflict(local, remote *engine.Task, winner *engine.Task) {
+func logConflict(local, remote *engine.Task, winnerLabel string) {
 	logPath, err := engine.GetSyncConflictLogPath()
 	if err != nil {
 		return
 	}
 
-	winnerLabel := "local"
-	if winner.UUID == remote.UUID && winner.Modified.Equal(remote.Modified) {
-		winnerLabel = "remote"
-	}
-
 	entry := fmt.Sprintf(
 		"[%s] Conflict on task %s\n"+
 			"  Local:  modified %s - %s\n"+