feat(FN-1479): restore projectId on subtask session rehydration

- Restore projectId field when rehydrating subtask sessions from SQLite via buildSubtaskSessionFromRow()
- Add regression tests for projectId forwarding in subtask start-streaming route
- Add documentation note in memory.md for durable subtask session context propagation

packages/dashboard/src/subtask-breakdown.ts

@@ -126,6 +126,7 @@ function buildSubtaskSessionFromRow(row: AiSessionRow): SubtaskInternalSession {
     createdAt,
     updatedAt,
     agent: undefined,
+    projectId: row.projectId ?? undefined,
   };
 }