feat(app): open composer subagent tabs in the explorer pane

Tapping a subagent or provider subagent from the composer track now opens its tab in the explorer companion pane when desktop split panes are active, keeping the parent agent visible in the primary pane.

Mobile and compact layouts continue using single-pane tab navigation, and cross-workspace subagents continue routing to their own workspace.

packages/app/src/panels/agent-tracks.tsx

@@ -1,8 +1,10 @@
 import { memo, useCallback, type ReactElement } from "react";
 import { AgentTaskList } from "@/composer/task-list";
 import { ComposerTrackBar } from "@/composer/tracks";
+import { supportsDesktopPaneSplits, useIsCompactFormFactor } from "@/constants/layout";
 import { usePaneContext } from "@/panels/pane-context";
 import { useSessionStore } from "@/stores/session-store";
+import { useWorkspaceLayoutStore } from "@/stores/workspace-layout-store";
 import {
   type ArchiveFinishedStatus,
   useArchiveSubagent,
@@ -12,6 +14,7 @@ import {
 import { SubagentsTrack } from "@/subagents/track";
 import type { TodoEntry } from "@/types/stream";
 import { navigateToAgent } from "@/utils/navigate-to-agent";
+import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
 
 /**
  * The pane's trackers — its subagents and its task list — as a row of pills above the composer.
@@ -33,7 +36,10 @@ export const AgentTracks = memo(function AgentTracks({
   archiveFinishedStatus: ArchiveFinishedStatus;
   onArchiveFinished: () => void;
 }): ReactElement | null {
-  const { openTab } = usePaneContext();
+  const { workspaceId, tabId, openTab } = usePaneContext();
+  const isCompact = useIsCompactFormFactor();
+  const canSplit = supportsDesktopPaneSplits() && !isCompact;
+  const workspaceKey = buildWorkspaceTabPersistenceKey({ serverId, workspaceId });
   const canDetachSubagents = useSessionStore(
     (state) => state.sessions[serverId]?.serverInfo?.features?.agentDetach === true,
   );
@@ -41,15 +47,35 @@ export const AgentTracks = memo(function AgentTracks({
   const detachSubagent = useDetachSubagent({ serverId });
   const handleOpenSubagent = useCallback(
     (subagentId: string) => {
+      const session = useSessionStore.getState().sessions[serverId];
+      const agent = session?.agents.get(subagentId) ?? session?.agentDetails.get(subagentId);
+      if (agent?.workspaceId && agent.workspaceId !== workspaceId) {
+        navigateToAgent({ serverId, agentId: subagentId });
+        return;
+      }
+      if (canSplit && workspaceKey) {
+        useWorkspaceLayoutStore.getState().openTabInExplorerPaneFocused(workspaceKey, {
+          target: { kind: "agent", agentId: subagentId },
+          parentTabId: tabId,
+        });
+        return;
+      }
       navigateToAgent({ serverId, agentId: subagentId });
     },
-    [serverId],
+    [canSplit, serverId, tabId, workspaceId, workspaceKey],
   );
   const handleOpenProviderSubagent = useCallback(
     (parentAgentId: string, subagentId: string) => {
+      if (canSplit && workspaceKey) {
+        useWorkspaceLayoutStore.getState().openTabInExplorerPaneFocused(workspaceKey, {
+          target: { kind: "provider_subagent", parentAgentId, subagentId },
+          parentTabId: tabId,
+        });
+        return;
+      }
       openTab({ kind: "provider_subagent", parentAgentId, subagentId });
     },
-    [openTab],
+    [canSplit, openTab, tabId, workspaceKey],
   );
 
   if (!hasAgentTracks({ subagentRows, tasks, archiveFinishedStatus })) {