Auto-attach pasted GitHub PR/issue URLs in the composer (#1006)

* Auto-attach pasted GitHub PR/issue URLs in the composer

Paste a GitHub PR or issue link into any composer and it shows up
as an attachment pill within a debounce tick when the URL matches
the workspace's remote. On the new-workspace screen, an attached PR
also surfaces an inline "Check out PR #N?" hint next to the picker;
one click promotes it to the checkout target through the same
syncPickerPrAttachment flow the dropdown uses.

Unifies the new-workspace picker and composer attachment picker on
one shared GitHub search hook and cache key, so both consumers (and
the auto-attach lookup) share the React Query cache.

* Fix oxfmt formatting in new-workspace-screen import

packages/app/src/components/composer-actions.ts

@@ -305,6 +305,29 @@ export function toggleGithubAttachment(
   return [...current, buildGithubAttachment(item)];
 }
 
+interface ToggleGithubAttachmentFromPickerInput {
+  current: UserComposerAttachment[];
+  item: GitHubSearchItem;
+  markGithubAttachmentRemoved: (attachment: UserComposerAttachment) => void;
+}
+
+export function toggleGithubAttachmentFromPicker({
+  current,
+  item,
+  markGithubAttachmentRemoved,
+}: ToggleGithubAttachmentFromPickerInput): UserComposerAttachment[] {
+  const existingAttachment = current.find(
+    (attachment) =>
+      attachment.kind !== "image" &&
+      attachment.item.kind === item.kind &&
+      attachment.item.number === item.number,
+  );
+  if (existingAttachment) {
+    markGithubAttachmentRemoved(existingAttachment);
+  }
+  return toggleGithubAttachment(current, item);
+}
+
 export function findGithubItemByOption(
   items: readonly GitHubSearchItem[],
   optionId: string,