perf(github): load the pull requests integration after start-up

Actions, the tool window and the timeline editor stay registered at start-up;
their implementations, the connection and review-mode tracking load on idle
or on first use. Takes ~60 KB gzip off the initial bundle.

src/features/github/shell.ts

@@ -0,0 +1,5 @@
+/** State `index.ts`'s registrations read before `runtime.ts` has loaded. */
+import { create } from "zustand";
+import type { MenuItem } from "@/ui";
+
+export const useGithubShell = create<{ headerMenu: MenuItem[] }>(() => ({ headerMenu: [] }));