refactor(surface): remove the main-area surface concept entirely

activeSurface was permanently 'chat' after the legacy mobile layout
removal, so the whole concept is gone: the store field, surfaceGuard,
setActiveSurface/setSurfaceGuard, the per-runtime surface memory in
prepare/restoreForRuntimeSwitch, and WorkspaceSurface itself. All ~30
setActiveSurface('chat') call sites were no-ops and are deleted;
always-true 'is the chat active' checks in keyboard shortcuts, Header
and ChatContainer are unconditional now. FilesView's dirty-file guard
kept its file-switch and close protection but drops the surface-switch
branch nothing could trigger. TerminalView visibility comes only from
its callers. The router keeps parsing legacy ?tab= links (they open the
matching context-panel surface) via its own RouteTab type and no longer
serializes a tab or diff file into URLs — desktop URLs never carried
them anyway.

packages/ui/src/lib/addSelectionToChat.ts

@@ -151,7 +151,6 @@ export const captureSelectionMarkdownForChat = (): string | null => {
 export const addSelectionToChat = (): boolean => {
   const markdown = captureSelectionMarkdownForChat();
 
-  useUIStore.getState().setActiveSurface('chat');
   useUIStore.getState().setSessionSwitcherOpen(false);
 
   if (markdown) {

packages/ui/src/lib/router/parseRoute.ts

@@ -1,6 +1,6 @@
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 import {
   type RouteState,
+  type RouteTab,
   VALID_TABS,
   VALID_SETTINGS_SECTIONS,
   ROUTE_PARAMS,
@@ -52,13 +52,13 @@ function parseSessionId(params: URLSearchParams): string | null {
  * Parse main tab from URL parameters.
  * Returns null if missing or invalid.
  */
-function parseTab(params: URLSearchParams): WorkspaceSurface | null {
+function parseTab(params: URLSearchParams): RouteTab | null {
   const value = params.get(ROUTE_PARAMS.TAB);
   if (!value) {
     return null;
   }
 
-  const normalized = value.toLowerCase().trim() as WorkspaceSurface;
+  const normalized = value.toLowerCase().trim() as RouteTab;
   if (VALID_TABS.includes(normalized)) {
     return normalized;
   }

packages/ui/src/lib/router/serializeRoute.test.ts

@@ -65,10 +65,8 @@ afterAll(() => {
 
 const sessionState = (sessionId: string): AppRouteState => ({
   sessionId,
-  tab: 'chat',
   isSettingsOpen: false,
   settingsPath: '',
-  diffFile: null,
 });
 
 describe('updateBrowserURL embedded-session-chat guard', () => {

packages/ui/src/lib/router/serializeRoute.ts

@@ -1,4 +1,3 @@
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 import { ROUTE_PARAMS } from './types';
 
@@ -7,17 +6,10 @@ import { ROUTE_PARAMS } from './types';
  */
 export interface AppRouteState {
   sessionId: string | null;
-  tab: WorkspaceSurface;
   isSettingsOpen: boolean;
   settingsPath: string;
-  diffFile: string | null;
 }
 
-/**
- * Default tab when none is specified.
- */
-const DEFAULT_TAB: WorkspaceSurface = 'chat';
-
 /**
  * Serialize application state to URL search parameters.
  * Only includes parameters that differ from defaults to keep URLs clean.
@@ -38,15 +30,6 @@ function serializeRoute(state: AppRouteState): URLSearchParams {
     return params;
   }
 
-  // Tab - only include if not the default
-  if (state.tab !== DEFAULT_TAB) {
-    params.set(ROUTE_PARAMS.TAB, state.tab);
-  }
-
-  // Diff file - only include when on diff tab
-  if (state.tab === 'diff' && state.diffFile && state.diffFile.trim().length > 0) {
-    params.set(ROUTE_PARAMS.FILE, state.diffFile);
-  }
 
   return params;
 }

packages/ui/src/lib/router/types.ts

@@ -1,5 +1,4 @@
 import type { SidebarSection } from '@/constants/sidebar';
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 
 /**
  * Represents the current route state derived from URL parameters.
@@ -9,7 +8,7 @@ export interface RouteState {
   /** Session ID to navigate to */
   sessionId: string | null;
   /** View selected through the legacy `tab` URL parameter. */
-  tab: WorkspaceSurface | null;
+  tab: RouteTab | null;
   /** Settings section - when non-null, settings dialog should be open */
   settingsPath: string | null;
   /** File path for diff view */
@@ -17,9 +16,11 @@ export interface RouteState {
 }
 
 /**
- * Valid values for the legacy `tab` URL parameter.
+ * Valid values for the legacy `tab` URL parameter. Non-chat tabs open the
+ * matching context-panel surface; the chat always owns the main area.
  */
-export const VALID_TABS: readonly WorkspaceSurface[] = ['chat', 'git', 'diff', 'terminal', 'files'] as const;
+export type RouteTab = 'chat' | 'git' | 'diff' | 'terminal' | 'files';
+export const VALID_TABS: readonly RouteTab[] = ['chat', 'git', 'diff', 'terminal', 'files'] as const;
 
 /**
  * Valid settings section values for URL routing.