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/agents/tools/sessions-spawn-tool.ts

@@ -70,12 +70,14 @@ export function createSessionsSpawnTool(opts?: {
   sandboxed?: boolean;
   /** Explicit agent ID override for cron/hook sessions where session key parsing may not work. */
   requesterAgentIdOverride?: string;
+  /** Internal-only workspace inheritance path for spawned subagents. */
+  workspaceDir?: string;
 }): AnyAgentTool {
   return {
     label: "Sessions",
     name: "sessions_spawn",
     description:
-      'Spawn an isolated session (runtime="subagent" or runtime="acp"). mode="run" is one-shot and mode="session" is persistent/thread-bound.',
+      'Spawn an isolated session (runtime="subagent" or runtime="acp"). mode="run" is one-shot and mode="session" is persistent/thread-bound. Subagents inherit the parent workspace directory automatically.',
     parameters: SessionsSpawnToolSchema,
     execute: async (_toolCallId, args) => {
       const params = args as Record<string, unknown>;
@@ -187,6 +189,7 @@ export function createSessionsSpawnTool(opts?: {
           agentGroupChannel: opts?.agentGroupChannel,
           agentGroupSpace: opts?.agentGroupSpace,
           requesterAgentIdOverride: opts?.requesterAgentIdOverride,
+          workspaceDir: opts?.workspaceDir,
         },
       );