feat(editor): page templates - live whole-page embed (MVP)

Embed another page's LIVE content into a host page (it updates when the source
changes, not a static copy). A page can be flagged a template for discovery in
the picker; any accessible page can be embedded.

Server:
- migrations: pages.is_template (+ partial index) and page_template_references
  (whole-page back-refs); db.d.ts/entity types hand-merged (db.d.ts is curated).
- POST /pages/toggle-template (CASL Edit) flips is_template; is_template is
  returned by findById + the sidebar tree select so the tree menu label
  reflects state. Search suggestions gain an onlyTemplates filter for the picker.
- POST /pages/template/lookup ({sourcePageIds[]}, <=50): returns each accessible
  source's {title, icon, slugId, content, sourceUpdatedAt} with comment marks
  stripped (same access path as transclusion: filterViewerAccessiblePageIds;
  inaccessible -> no_access, missing -> not_found; error path -> not_found, never
  raw content).
- reference sync (collectPageEmbedsFromPmJson + syncPageTemplateReferences) on
  the Yjs save hook; duplicatePage remaps pageEmbed.sourcePageId + inserts refs.
  Known MVP gap: REST content updates don't resync refs (lookup uses in-doc ids).

Client:
- pageEmbed node (editor-ext, registered in BOTH client + server schemas);
  read-only NodeView with a batching lookup; '/Embed page' slash + template
  picker (self-embed prevented); 'Make/Unset template' in the tree node menu.
- Cycle guard: an ancestry-chain context + depth cap (5) render a 'circular
  embed' placeholder instead of recursing.
- Public shares show a placeholder (no public lookup in MVP).

MVP excludes (follow-ups): public-share lookup, unsync->static copy, server-side
expansion for export/RAG, MCP schema mirror, point-in-time snapshots.

Implements docs/page-templates-plan.md (MVP, variant A).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/editor-ext/src/index.ts

@@ -22,6 +22,7 @@ export * from "./lib/search-and-replace";
 export * from "./lib/embed-provider";
 export * from "./lib/subpages";
 export * from "./lib/transclusion";
+export * from "./lib/page-embed";
 export * from "./lib/highlight";
 export * from "./lib/indent";
 export * from "./lib/heading/heading";

packages/editor-ext/src/lib/page-embed/index.ts

@@ -0,0 +1 @@
+export * from "./page-embed";

packages/editor-ext/src/lib/page-embed/page-embed.ts

@@ -0,0 +1,88 @@
+import { mergeAttributes, Node } from "@tiptap/core";
+import { ReactNodeViewRenderer } from "@tiptap/react";
+
+export interface PageEmbedOptions {
+  HTMLAttributes: Record<string, any>;
+  view: any;
+}
+
+export interface PageEmbedAttributes {
+  sourcePageId?: string | null;
+}
+
+declare module "@tiptap/core" {
+  interface Commands<ReturnType> {
+    pageEmbed: {
+      insertPageEmbed: (attributes: PageEmbedAttributes) => ReturnType;
+    };
+  }
+}
+
+/**
+ * Whole-page live embed. Holds only a `sourcePageId` reference; the node view
+ * fetches the source page's current content at render time, so the embed stays
+ * live (no snapshot is stored in the host document). Separate from
+ * `transclusionReference` (which addresses a single block by `transclusionId`).
+ */
+export const PageEmbed = Node.create<PageEmbedOptions>({
+  name: "pageEmbed",
+
+  addOptions() {
+    return {
+      HTMLAttributes: {},
+      view: null,
+    };
+  },
+
+  group: "block",
+  atom: true,
+  isolating: true,
+  selectable: true,
+  draggable: true,
+
+  addAttributes() {
+    return {
+      sourcePageId: {
+        default: null,
+        parseHTML: (el) => el.getAttribute("data-source-page-id"),
+        renderHTML: (attrs) =>
+          attrs.sourcePageId
+            ? { "data-source-page-id": attrs.sourcePageId }
+            : {},
+      },
+    };
+  },
+
+  parseHTML() {
+    return [{ tag: `div[data-type="${this.name}"]` }];
+  },
+
+  renderHTML({ HTMLAttributes }) {
+    return [
+      "div",
+      mergeAttributes(
+        { "data-type": this.name },
+        this.options.HTMLAttributes,
+        HTMLAttributes,
+      ),
+    ];
+  },
+
+  addCommands() {
+    return {
+      insertPageEmbed:
+        (attributes) =>
+        ({ commands }) =>
+          commands.insertContent({
+            type: this.name,
+            attrs: attributes,
+          }),
+    };
+  },
+
+  addNodeView() {
+    if (!this.options.view) return null;
+    this.editor.isInitialized = true;
+    return ReactNodeViewRenderer(this.options.view);
+  },
+});