refactor(app): replace screen test slop batch 2 with proper coverage (#725)

Deletes four screen-level .test.tsx files (2600+ lines of vi.mock + JSDOM
slop) and replaces each with verified coverage:

- sessions-screen.test.tsx: covered by archive-tab.spec.ts which exercises
  the sessions screen via openSessions() with real agents
- workspace-draft-agent-tab.test.tsx: covered by new-workspace.spec.ts
  "redirects to optimistic draft tab before agent creation resolves"
- new-workspace-screen.test.tsx: covered by new-workspace.spec.ts (main
  submit, branch selection, PR selection, optimistic draft tab) plus
  new-workspace-picker-item.test.ts (pickerItemToCheckoutRequest)
- project-settings-screen.test.tsx: covered by project-config-form.test.ts
  (all round-trip string/array lifecycle semantics) and projects-settings.spec.ts
  (save flow with passthrough field preservation)

Extracts syncPickerPrAttachment from new-workspace-screen.tsx into a new
pure module new-workspace-picker-state.ts with 5 zero-mock unit tests
covering: initial PR selection, branch selection without change, PR
replacement, PR removal on branch switch, and no-duplicate guard.

packages/app/src/screens/new-workspace-screen.tsx

@@ -31,6 +31,7 @@ import type { ImageAttachment, MessagePayload } from "@/components/message-input
 import type { AgentAttachment, GitHubSearchItem } from "@server/shared/messages";
 import type { AgentProvider } from "@server/server/agent/agent-sdk-types";
 import { pickerItemToCheckoutRequest, type PickerItem } from "./new-workspace-picker-item";
+import { syncPickerPrAttachment } from "./new-workspace-picker-state";
 
 interface NewWorkspaceScreenProps {
   serverId: string;
@@ -194,36 +195,6 @@ function pickerItemTriggerLabel(item: PickerItem): string {
   return item.kind === "branch" ? item.name : formatPrLabel(item.item);
 }
 
-function syncPickerPrAttachment(input: {
-  attachments: UserComposerAttachment[];
-  previousPickerPrNumber: number | null;
-  item: PickerItem;
-}): { attachments: UserComposerAttachment[]; attachedPrNumber: number | null } {
-  let nextAttachments = input.attachments;
-  let attachedPrNumber: number | null = null;
-
-  if (input.previousPickerPrNumber !== null) {
-    nextAttachments = nextAttachments.filter(
-      (attachment) =>
-        attachment.kind !== "github_pr" || attachment.item.number !== input.previousPickerPrNumber,
-    );
-  }
-
-  if (input.item.kind === "github-pr") {
-    const selectedPr = input.item.item;
-    const hasExistingPrAttachment = nextAttachments.some(
-      (attachment) =>
-        attachment.kind === "github_pr" && attachment.item.number === selectedPr.number,
-    );
-    if (!hasExistingPrAttachment) {
-      nextAttachments = [...nextAttachments, { kind: "github_pr", item: selectedPr }];
-      attachedPrNumber = selectedPr.number;
-    }
-  }
-
-  return { attachments: nextAttachments, attachedPrNumber };
-}
-
 function computePickerOptionData(
   branchDetails: ReadonlyArray<{ name: string; committerDate: number }>,
   prItems: ReadonlyArray<GitHubSearchItem>,