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/test/schema-surface-snapshot.test.ts

@@ -99,7 +99,7 @@ const expectedSurface: SurfaceEntry[] = [
   { name: "pdf", kind: "node", attrs: ["attachmentId", "height", "name", "placeholder", "size", "src", "width"] },
   { name: "status", kind: "node", attrs: ["color", "text"] },
   { name: "strike", kind: "mark", attrs: [] },
-  { name: "subpages", kind: "node", attrs: [] },
+  { name: "subpages", kind: "node", attrs: ["recursive"] },
   { name: "subscript", kind: "mark", attrs: [] },
   { name: "superscript", kind: "mark", attrs: [] },
   { name: "table", kind: "node", attrs: [] },