feat(git-sync): phase 2b — PULL writes native gitmost_id frontmatter

PULL now serializes each page as the native-Obsidian format (serializePageFile:
a minimal gitmost_id frontmatter + the fixpoint markdown body) instead of the
heavy docmost:meta envelope. title/parent/space are derived (filename / folder /
repo), so only the pageId is persisted. readExisting recovers identity from the
gitmost_id frontmatter (parsePageFile) instead of docmost:meta.

Extracted stabilizePageBody() (the export->import->export fixpoint, no meta) so
the native writer and the legacy serializer share the same deterministic body —
re-pulls of an unchanged page stay byte-identical (loop-guard).

Tests: read-existing fixtures rewritten to gitmost_id; apply-pull asserts the
written text is native frontmatter and carries NO docmost:meta (regression
guard). 611 engine tests green.

NOTE: PUSH still reads docmost:meta — the end-to-end cycle is intentionally NOT
runnable until phase 3 (PUSH reads frontmatter + derives title/parent from path)
lands; no vault is wiped/deployed until then.

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

packages/git-sync/test/apply-pull-actions.test.ts

@@ -164,6 +164,14 @@ describe('applyPullActions — happy path (write + commit + merge)', () => {
     const writtenPaths = fs.writes.map((w) => w.abs).sort();
     expect(writtenPaths).toEqual(['/vault/A.md', '/vault/Sub/B.md']);
 
+    // Every written file is in the native-Obsidian format: a `gitmost_id`
+    // frontmatter at the very top and NO legacy `docmost:meta` envelope. Guards
+    // against a regression back to the heavy meta block.
+    for (const w of fs.writes) {
+      expect(w.text.startsWith('---\ngitmost_id: ')).toBe(true);
+      expect(w.text).not.toContain('docmost:meta');
+    }
+
     // The git op order is: stageAll -> commit -> checkout main -> merge.
     expect(g.order).toEqual([
       'stageAll',