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/components/views/git/PullRequestSection.tsx

@@ -327,7 +327,6 @@ export const PullRequestSection: React.FC<{
   const githubAuthChecked = useGitHubAuthStore((state) => state.hasChecked);
   const setSettingsDialogOpen = useUIStore((state) => state.setSettingsDialogOpen);
   const setSettingsPage = useUIStore((state) => state.setSettingsPage);
-  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const currentSessionId = useSessionUIStore((state) => state.currentSessionId);
   const newSessionDraftOpen = useSessionUIStore((state) => Boolean(state.newSessionDraft?.open));
   const { isMobile, hasTouchInput, screenWidth } = useDeviceInfo();
@@ -986,14 +985,13 @@ export const PullRequestSection: React.FC<{
           text: '',
         });
       }
-      setActiveSurface('chat');
     } catch (e) {
       const message = e instanceof Error ? e.message : String(e);
       toast.error(t('gitView.pr.toast.loadChecksFailed'), { description: message });
     } finally {
       setIsAttachingChecks(false);
     }
-  }, [directory, ensurePrContext, github, pr, resolveDraftTarget, setActiveSurface, status?.repo, t]);
+  }, [directory, ensurePrContext, github, pr, resolveDraftTarget, status?.repo, t]);
 
   const sendCommentsToChat = React.useCallback(async () => {
     if (!github?.prContext) {
@@ -1021,14 +1019,13 @@ export const PullRequestSection: React.FC<{
       for (const comment of timelineComments) {
         attachCommentDraft(target, comment);
       }
-      setActiveSurface('chat');
     } catch (e) {
       const message = e instanceof Error ? e.message : String(e);
       toast.error(t('gitView.pr.toast.loadPrCommentsFailed'), { description: message });
     } finally {
       setIsAttachingComments(false);
     }
-  }, [attachCommentDraft, directory, ensurePrContext, github, pr, resolveDraftTarget, setActiveSurface, status?.repo, t, timelineComments]);
+  }, [attachCommentDraft, directory, ensurePrContext, github, pr, resolveDraftTarget, status?.repo, t, timelineComments]);
 
   const sendSingleCommentToChat = React.useCallback(async (comment: TimelineCommentItem) => {
     const target = resolveDraftTarget();
@@ -1037,8 +1034,7 @@ export const PullRequestSection: React.FC<{
     }
 
     attachCommentDraft(target, comment);
-    setActiveSurface('chat');
-  }, [attachCommentDraft, resolveDraftTarget, setActiveSurface]);
+  }, [attachCommentDraft, resolveDraftTarget]);
 
   const refresh = React.useCallback(async (options?: { force?: boolean; onlyExistingPr?: boolean; silent?: boolean; markInitialResolved?: boolean }) => {
     await refreshPrStatus(prStatusKey, options);