Expand workspace panel actions and keyboard navigation (#3685)

* fix(app): keep file navigation in place

Retargeting a File tab allocated a new tab identity, causing the tab track to unmount and remount during navigation. Preserve the identity for File-to-File changes while retaining new instances for cross-kind replacements.

* feat(app): expand workspace panel actions

Keep the Command Center aligned with workspace tab and pane primitives. Add explicit panel placement, focused open behavior, keyboard navigation for empty panes, and a Changes pill that toggles the visible Side panel without closing its tab.

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

@@ -17,7 +17,7 @@ import { SubagentsTrack } from "@/subagents/track";
 import type { TodoEntry } from "@/types/stream";
 import { navigateToAgent } from "@/utils/navigate-to-agent";
 import { buildWorkspaceTabPersistenceKey } from "@/workspace-tabs/model";
-import { openSupportingTab } from "@/workspace-tabs/side-panel";
+import { openSupportingTab, toggleSupportingTab } from "@/workspace-tabs/side-panel";
 
 /**
  * The pane's ambient context — workspace changes, subagents, and tasks — as a row of pills above
@@ -96,7 +96,7 @@ export const AgentTracks = memo(function AgentTracks({
     if (!workspaceKey) {
       return;
     }
-    openSupportingTab({
+    toggleSupportingTab({
       isCompact,
       workspaceKey,
       target: { kind: "working_diff" },