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-screen.tsx

@@ -33,7 +33,10 @@ import type { AgentAttachment, GitHubSearchItem } from "@server/shared/messages"
 import type { AgentProvider } from "@server/server/agent/agent-sdk-types";
 import { isEmptyWorkspaceSubmission, runCreateEmptyWorkspace } from "./new-workspace-empty";
 import { pickerItemToCheckoutRequest, type PickerItem } from "./new-workspace-picker-item";
-import { syncPickerPrAttachment } from "./new-workspace-picker-state";
+import {
+  deriveAutoPickerItemFromAttachments,
+  syncPickerPrAttachment,
+} from "./new-workspace-picker-state";
 
 interface NewWorkspaceScreenProps {
   serverId: string;
@@ -51,6 +54,17 @@ interface PickerSelection {
   attachedPrNumber: number | null;
 }
 
+// Manual picks always win; the auto-promoted item is a fallback so the user
+// doesn't silently get "main" when they meant the PR they just attached.
+function combinePickerSelection(
+  manual: PickerSelection | null,
+  autoItem: PickerItem | null,
+): PickerSelection | null {
+  if (manual) return manual;
+  if (autoItem) return { item: autoItem, attachedPrNumber: null };
+  return null;
+}
+
 const BRANCH_OPTION_PREFIX = "branch:";
 const PR_OPTION_PREFIX = "github-pr:";
 
@@ -400,7 +414,7 @@ export function NewWorkspaceScreen({
     typeof normalizeWorkspaceDescriptor
   > | null>(null);
   const [pendingAction, setPendingAction] = useState<"chat" | "empty" | null>(null);
-  const [pickerSelection, setPickerSelection] = useState<PickerSelection | null>(null);
+  const [manualPickerSelection, setManualPickerSelection] = useState<PickerSelection | null>(null);
   const [pickerOpen, setPickerOpen] = useState(false);
   const [pickerSearchQuery, setPickerSearchQuery] = useState("");
   const [debouncedPickerSearchQuery, setDebouncedPickerSearchQuery] = useState("");
@@ -414,7 +428,6 @@ export function NewWorkspaceScreen({
 
   const displayName = displayNameProp?.trim() ?? "";
   const workspace = createdWorkspace;
-  const selectedItem = pickerSelection?.item ?? null;
   const isPending = pendingAction !== null;
   const client = useHostRuntimeClient(serverId);
   const isConnected = useHostRuntimeIsConnected(serverId);
@@ -430,6 +443,13 @@ export function NewWorkspaceScreen({
   });
   const composerState = chatDraft.composerState;
 
+  const autoPickerItem = useMemo(
+    () => deriveAutoPickerItemFromAttachments(chatDraft.attachments),
+    [chatDraft.attachments],
+  );
+  const pickerSelection = combinePickerSelection(manualPickerSelection, autoPickerItem);
+  const selectedItem = pickerSelection?.item ?? null;
+
   const withConnectedClient = useCallback(() => {
     if (!client || !isConnected) {
       throw new Error("Host is not connected");
@@ -525,7 +545,7 @@ export function NewWorkspaceScreen({
         item,
       });
 
-      setPickerSelection({
+      setManualPickerSelection({
         item,
         attachedPrNumber: next.attachedPrNumber,
       });