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-picker-state.ts

@@ -0,0 +1,35 @@
+import type { UserComposerAttachment } from "@/attachments/types";
+import type { PickerItem } from "./new-workspace-picker-item";
+
+// The picker "owns" at most one PR attachment at a time. When the user selects
+// a different item the previously-owned PR is removed before the new one is added.
+// User-added attachments for other PRs/issues are left untouched.
+export 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 };
+}