fix(agents): land #39247 from @jasonQin6 (subagent workspace inheritance)

Propagate parent workspace directories into spawned subagent runs, keep workspace override internal-only, and add regression tests for forwarding boundaries.

Co-authored-by: jasonQin6 <991262382@qq.com>

src/gateway/protocol/schema/agent.ts

@@ -110,6 +110,7 @@ export const AgentParamsSchema = Type.Object(
     idempotencyKey: NonEmptyString,
     label: Type.Optional(SessionLabelString),
     spawnedBy: Type.Optional(Type.String()),
+    workspaceDir: Type.Optional(Type.String()),
   },
   { additionalProperties: false },
 );