fix(git-sync): preserve subpages.recursive and details.open on round trip

Found proactively by deepening the round-trip test from node-TYPE survival to
ATTRIBUTE fidelity (distinctive attr values per node). Two real losses (the
other 3 candidates — mathInline/mathBlock/pageEmbed — were verified to be
correct; the probe had used wrong attr names):

- subpages `recursive`: the converter emitted a bare div and the schema mirror
  didn't model the attr, so a recursive subpages reverted to non-recursive on a
  round trip. Now emits `data-recursive="true"` and the mirror parses it back
  (matching @docmost/editor-ext).
- details `open`: the `open` (collapsed/expanded) state lives on the details
  node, but the converter emitted the `<details>` wrapper from the summary case
  without it, so the state was dropped. The wrapper now carries `open`.

The round-trip test now also asserts attribute fidelity (12 cases) so these are
locked. Schema-surface snapshot updated for the new subpages attr.

git-sync vitest 671 (+1 expected-fail), §13.1 gate 27.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/git-sync/src/lib/docmost-schema.ts

@@ -928,6 +928,17 @@ const Subpages = Node.create({
   atom: true,
   defining: true,
   draggable: true,
+  addAttributes() {
+    return {
+      recursive: {
+        default: false,
+        parseHTML: (el: HTMLElement) =>
+          el.getAttribute("data-recursive") === "true",
+        renderHTML: (attrs: Record<string, any>) =>
+          attrs.recursive ? { "data-recursive": "true" } : {},
+      },
+    };
+  },
   parseHTML() {
     return [{ tag: 'div[data-type="subpages"]' }];
   },