fix(app): reduce unnecessary workspace screen re-renders from unstable callback deps

- Add useCloseTabs hook to centralize tab close state with stable closeTab callback and reactive closingTabIds
- Replace killTerminalMutation/isArchivingAgent props with closingTabIds Set in split container
- Stabilize archiveAgent callback by depending on mutateAsync instead of whole mutation object
- Add bail-out to focusTabInLayout and focusPaneInLayout when already focused
- Remove redundant isArchivingAgent guard (closeTab handles dedup)

packages/app/src/stores/workspace-layout-actions.ts

@@ -995,6 +995,10 @@ export function focusTabInLayout(input: FocusTabInLayoutInput): WorkspaceLayout
     return null;
   }
 
+  if (pane.focusedTabId === input.tabId && layout.focusedPaneId === pane.id) {
+    return null;
+  }
+
   return {
     root: focusTabInPane(layout.root, pane.id, input.tabId),
     focusedPaneId: pane.id,
@@ -1157,6 +1161,9 @@ export function focusPaneInLayout(input: FocusPaneInLayoutInput): WorkspaceLayou
   if (!findPaneById(input.layout.root, input.paneId)) {
     return null;
   }
+  if (input.layout.focusedPaneId === input.paneId) {
+    return null;
+  }
   return {
     root: input.layout.root,
     focusedPaneId: input.paneId,