refactor(surface): drop the deprecated main-tab aliases and dead diagram surface

MainTab/activeMainTab/setActiveMainTab/setMainTabGuard were deprecated
mirrors of the surface names — every call site now uses
activeSurface/setActiveSurface/setSurfaceGuard directly and the aliases
are gone, including the persisted mirror field.

The 'diagram' surface had no way to open it (navigateToDiagram had no
callers except a .drawio attachment click that navigated to a surface
nothing rendered); the surface, DiagramView, and its store plumbing are
removed, and a .drawio attachment now opens in the file panel.

?tab= deep links map to the matching context-panel surface instead of
setting a main-area surface nothing renders, and a persisted non-chat
surface can no longer rehydrate into a blank main area.

packages/ui/src/lib/addSelectionToChat.test.ts

@@ -2,7 +2,7 @@ import { afterEach, beforeEach, describe, expect, mock, test } from 'bun:test';
 
 const focusChatInputCalls: number[] = [];
 const pendingInputCalls: Array<{ text: string | null; mode?: string }> = [];
-const activeMainTabCalls: string[] = [];
+const activeSurfaceCalls: string[] = [];
 const sessionSwitcherCalls: boolean[] = [];
 const codeMirrorDispatches: Array<{ selection: { anchor: number } }> = [];
 
@@ -41,8 +41,8 @@ mock.module('@/sync/input-store', () => ({
 mock.module('@/stores/useUIStore', () => ({
   useUIStore: {
     getState: () => ({
-      setActiveMainTab: (tab: string) => {
-        activeMainTabCalls.push(tab);
+      setActiveSurface: (tab: string) => {
+        activeSurfaceCalls.push(tab);
       },
       setSessionSwitcherOpen: (open: boolean) => {
         sessionSwitcherCalls.push(open);
@@ -86,7 +86,7 @@ const installSelectionEnvironment = (options: {
 const clearCalls = () => {
   focusChatInputCalls.length = 0;
   pendingInputCalls.length = 0;
-  activeMainTabCalls.length = 0;
+  activeSurfaceCalls.length = 0;
   sessionSwitcherCalls.length = 0;
   codeMirrorDispatches.length = 0;
   codeMirrorView = null;
@@ -262,7 +262,7 @@ describe('addSelectionToChat', () => {
     installSelectionEnvironment({ activeElement: textarea });
 
     expect(addSelectionToChat()).toBe(true);
-    expect(activeMainTabCalls).toEqual(['chat']);
+    expect(activeSurfaceCalls).toEqual(['chat']);
     expect(sessionSwitcherCalls).toEqual([false]);
     expect(pendingInputCalls).toEqual([{ text: '```md\nselected\n```', mode: 'append' }]);
 
@@ -290,7 +290,7 @@ describe('addSelectionToChat', () => {
 
     expect(addSelectionToChat()).toBe(false);
     expect(pendingInputCalls).toEqual([]);
-    expect(activeMainTabCalls).toEqual(['chat']);
+    expect(activeSurfaceCalls).toEqual(['chat']);
 
     await Promise.resolve();
     expect(focusChatInputCalls.length).toBe(1);

packages/ui/src/lib/addSelectionToChat.ts

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

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

@@ -19,7 +19,7 @@ export interface RouteState {
 /**
  * Valid values for the legacy `tab` URL parameter.
  */
-export const VALID_TABS: readonly WorkspaceSurface[] = ['chat', 'git', 'diff', 'terminal', 'files', 'diagram'] as const;
+export const VALID_TABS: readonly WorkspaceSurface[] = ['chat', 'git', 'diff', 'terminal', 'files'] as const;
 
 /**
  * Valid settings section values for URL routing.