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/api/handlers/sync.go

@@ -104,6 +104,8 @@ func PushChanges(w http.ResponseWriter, r *http.Request) {
 			if err := task.Save(); err != nil {
 				continue
 			}
+			// Mark as sync-originated to prevent feedback loop
+			_ = engine.MarkChangeLogAsSync(task.UUID.String())
 			// Add tags
 			for _, tag := range task.Tags {
 				_ = task.AddTag(tag)
@@ -114,15 +116,18 @@ func PushChanges(w http.ResponseWriter, r *http.Request) {
 
 		// Task exists - check timestamps for conflicts
 		if existing.Modified.Unix() > task.Modified.Unix() {
-			// Server version is newer - conflict (but we'll apply last-write-wins)
+			// Server version is newer - skip this push
 			conflicts++
+			continue
 		}
 
-		// Apply changes (last-write-wins)
+		// Apply changes (client is newer or equal)
 		task.ID = existing.ID // Preserve database ID
 		if err := task.Save(); err != nil {
 			continue
 		}
+		// Mark as sync-originated to prevent feedback loop
+		_ = engine.MarkChangeLogAsSync(task.UUID.String())
 
 		// Sync tags
 		existingTags := make(map[string]bool)