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/engine/database.go

@@ -307,6 +307,13 @@ func runMigrations() error {
 				END;
 			`,
 		},
+		{
+			version: 2,
+			sql: `
+				ALTER TABLE change_log ADD COLUMN source TEXT NOT NULL DEFAULT 'local';
+				CREATE INDEX idx_change_log_source ON change_log(source);
+			`,
+		},
 	}
 
 	// Apply pending migrations
@@ -409,3 +416,24 @@ func SetChangeLogRetentionDays(days int) error {
 	_, err := db.Exec("INSERT OR REPLACE INTO sync_config (key, value) VALUES ('change_log_retention_days', ?)", days)
 	return err
 }
+
+// MarkChangeLogAsSync marks the most recent change_log entry for a task UUID
+// as originating from sync (not local), preventing the feedback loop where
+// synced changes get re-pushed as local changes.
+func MarkChangeLogAsSync(taskUUID string) error {
+	db := GetDB()
+	if db == nil {
+		return fmt.Errorf("database not initialized")
+	}
+
+	_, err := db.Exec(`
+		UPDATE change_log SET source = 'sync'
+		WHERE id = (
+			SELECT id FROM change_log
+			WHERE task_uuid = ?
+			ORDER BY id DESC
+			LIMIT 1
+		)
+	`, taskUUID)
+	return err
+}