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/commands/agent.ts

@@ -539,7 +539,9 @@ async function agentCommandInternal(
     agentId: sessionAgentId,
     sessionKey,
   });
-  const workspaceDirRaw = resolveAgentWorkspaceDir(cfg, sessionAgentId);
+  // Internal callers (for example subagent spawns) may pin workspace inheritance.
+  const workspaceDirRaw =
+    opts.workspaceDir?.trim() ?? resolveAgentWorkspaceDir(cfg, sessionAgentId);
   const agentDir = resolveAgentDir(cfg, sessionAgentId);
   const workspace = await ensureAgentWorkspace({
     dir: workspaceDirRaw,

src/commands/agent/types.ts

@@ -80,6 +80,8 @@ export type AgentCommandOpts = {
   inputProvenance?: InputProvenance;
   /** Per-call stream param overrides (best-effort). */
   streamParams?: AgentStreamParams;
+  /** Explicit workspace directory override (for subagents to inherit parent workspace). */
+  workspaceDir?: string;
 };
 
 export type AgentCommandIngressOpts = Omit<AgentCommandOpts, "senderIsOwner"> & {