fix(app): auto-set ref picker from a single attached PR (#951)

* fix(app): auto-set ref picker from a single attached PR

Attaching a PR in the new-workspace composer signals "I want this PR's
context", and almost always also means "base the worktree on this PR".
Until now the two paths were independent: the ref picker stayed on the
default branch, so the worktree was branched off main even though the
sidebar chip looked like a PR checkout.

Auto-promote the picker when exactly one PR is attached and the user
hasn't manually picked a ref. Manual picker selections are preserved;
auto-promoted ones are cleared when their backing attachment is removed.

* refactor(app): derive auto-promoted picker selection instead of syncing

The previous version drove `pickerSelection` from a useEffect that
inspected attachments and overwrote state. That meant the shared
`PickerSelection` type had to grow a `source: "manual" | "attachment"`
discriminator just so the effect could tell its own writes apart from
manual picks. Classic effect-as-derived-state.

Split the state instead. `manualPickerSelection` holds explicit picks;
`autoPickerItem` is a useMemo over composer attachments; the effective
`pickerSelection` is `manual ?? auto` via a small precedence helper. No
effect, no `source` flag, and `PickerSelection` stays a screen-local
shape since nothing outside needs it.

packages/app/src/screens/new-workspace-picker-state.test.ts

@@ -1,7 +1,10 @@
 import { describe, expect, it } from "vitest";
 import type { UserComposerAttachment } from "@/attachments/types";
 import type { GitHubSearchItem } from "@server/shared/messages";
-import { syncPickerPrAttachment } from "./new-workspace-picker-state";
+import {
+  deriveAutoPickerItemFromAttachments,
+  syncPickerPrAttachment,
+} from "./new-workspace-picker-state";
 
 function makePrItem(number: number, title: string, headRefName = "feature/x"): GitHubSearchItem {
   return {
@@ -94,3 +97,35 @@ describe("syncPickerPrAttachment", () => {
     expect(result.attachments).toHaveLength(1);
   });
 });
+
+describe("deriveAutoPickerItemFromAttachments", () => {
+  it("returns null when there are no attachments", () => {
+    expect(deriveAutoPickerItemFromAttachments([])).toBeNull();
+  });
+
+  it("returns the PR when exactly one is attached", () => {
+    const pr = makePrItem(923, "Nix overridable npm deps hash");
+    expect(deriveAutoPickerItemFromAttachments([prAttachment(pr)])).toEqual({
+      kind: "github-pr",
+      item: pr,
+    });
+  });
+
+  it("returns null when multiple PRs are attached", () => {
+    const a = makePrItem(101, "A");
+    const b = makePrItem(202, "B");
+    expect(deriveAutoPickerItemFromAttachments([prAttachment(a), prAttachment(b)])).toBeNull();
+  });
+
+  it("ignores non-PR attachments", () => {
+    expect(deriveAutoPickerItemFromAttachments([issueAttachment(44)])).toBeNull();
+  });
+
+  it("returns the lone PR even when other non-PR attachments are present", () => {
+    const pr = makePrItem(923, "Nix overridable npm deps hash");
+    expect(deriveAutoPickerItemFromAttachments([issueAttachment(44), prAttachment(pr)])).toEqual({
+      kind: "github-pr",
+      item: pr,
+    });
+  });
+});