Gateway: keep spawned workspace overrides internal (#43801)

* Gateway: keep spawned workspace overrides internal

* Changelog: note GHSA-2rqg agent boundary fix

* Gateway: persist spawned workspace inheritance in sessions

* Agents: clean failed lineage spawn state

* Tests: cover lineage attachment cleanup

* Tests: cover lineage thread cleanup

src/config/sessions/types.ts

@@ -78,6 +78,8 @@ export type SessionEntry = {
   sessionFile?: string;
   /** Parent session key that spawned this session (used for sandbox session-tool scoping). */
   spawnedBy?: string;
+  /** Workspace inherited by spawned sessions and reused on later turns for the same child session. */
+  spawnedWorkspaceDir?: string;
   /** True after a thread/topic session has been forked from its parent transcript once. */
   forkedFromParent?: boolean;
   /** Subagent spawn depth (0 = main, 1 = sub-agent, 2 = sub-sub-agent). */