the vendoring machinery: one command reproduces the whole state

scripts/vendor.sh clones upstream, rewrites @/ imports to relative
paths, and applies scripts/apply-host.py. web/host.ts is ours and gets
placed into the vendored tree, because upstream declined it.

a script rather than a .patch, and that is not a preference. between
cfde698 and 5c83ac6 — one day — upstream changed an import line,
re-indented the component from 6 spaces to 8, and added an editAccess
guard. a context diff fails on all three, none of which concern us. so
every anchor matches structure with flexible whitespace and re-indents
to whatever it finds, and a missing anchor stops the run loudly.

all three prompt-export paths are wired, which is what upstream said a
complete version needs. the other two clipboard writes are ShareMenu's
share-link and ask-an-AI blurb, and the reason they are excluded is
written down so the count does not read as the same oversight twice.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/host.ts

@@ -0,0 +1,46 @@
+"use client";
+
+import { createContext, useContext } from "react";
+import type { Doc } from "./tokens";
+
+/**
+ * Hooks for an application that embeds the editor.
+ *
+ * The editor keeps its own state and its own UI; a host only needs to say where the document is kept
+ * and where the prompt goes. Every field is optional and every default is what the standalone app
+ * already did — localStorage for the document, the clipboard for the prompt — so an editor rendered
+ * without a provider behaves exactly as before.
+ *
+ * ```tsx
+ * <HostProvider value={{ loadDoc, saveDoc, onExport: (text) => sendToAgent(text) }}>
+ *   <Canvas />
+ * </HostProvider>
+ * ```
+ */
+export type HostAdapter = {
+  /** Return the saved document, or null for a fresh one. Omit to read localStorage. */
+  loadDoc?: () => Partial<Doc> | null;
+  /** Persist the document. Omit to write localStorage. Called on every change, so debounce if remote. */
+  saveDoc?: (doc: Partial<Doc>) => void;
+
+  /** Editor chrome — panel widths, favourites. Separate from the document because it is not the work. */
+  loadUi?: () => unknown | null;
+  saveUi?: (ui: unknown) => void;
+
+  /**
+   * Where the generated prompt goes.
+   *
+   * Return `true` to say the host has handled it and the editor should not also write the clipboard;
+   * anything else and the clipboard write still happens. That default matters — a host that only
+   * wants to observe the export does not have to reimplement copying.
+   */
+  onExport?: (prompt: string, doc: Doc) => boolean | void;
+};
+
+const EMPTY: HostAdapter = {};
+
+export const HostContext = createContext<HostAdapter>(EMPTY);
+
+export const HostProvider = HostContext.Provider;
+
+export const useHost = (): HostAdapter => useContext(HostContext);