feat(git-sync): vendor pure converter + engine into @docmost/git-sync (Phase A.1)

First step of docs/git-sync-plan.md. New workspace package @docmost/git-sync
vendoring the PURE parts from docmost-sync (HEAD b03eb35):
- lib: markdown-converter, markdown-document, canonicalize, docmost-schema,
  node-ops, diff, and an extracted markdown-to-prosemirror (only the pure
  marked->HTML->generateJSON path from upstream collaboration.ts; no websocket).
- engine (pure, no IO): reconcile, layout, sanitize, stabilize, loop-guard.
Ported the upstream pure-module + round-trip corpus tests (vitest): 314 pass,
3 expected upstream known-limitation fails. tsc clean. No server wiring yet.

docmost-schema inlines getStyleProperty (as packages/mcp does — @tiptap/core
3.20.4 doesn't export it). IO engine (pull/push/git/settings) deferred to later
Phase A/B steps; the editor-ext idempotency gate (plan §13.1) is the next step.

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

packages/git-sync/test/markdown-document.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest';
+import {
+  serializeDocmostMarkdownBody,
+  parseDocmostMarkdown,
+  type DocmostMdMeta,
+} from 'docmost-client';
+
+describe('serializeDocmostMarkdownBody round-trip (SPEC §3)', () => {
+  it('serialize -> parse preserves meta and the trimmed body, with no comments block', () => {
+    const meta: DocmostMdMeta = {
+      version: 1,
+      pageId: 'page-123',
+      slugId: 'slug-abc',
+      title: 'My Page',
+      spaceId: 'space-1',
+      parentPageId: 'parent-9',
+    };
+    const body = 'Hello\n\nWorld';
+
+    const file = serializeDocmostMarkdownBody(meta, body);
+    const parsed = parseDocmostMarkdown(file);
+
+    expect(parsed.meta).toEqual(meta);
+    expect(parsed.body).toBe(body);
+    // No trailing docmost:comments block was emitted (SPEC §3).
+    expect(parsed.comments).toBeNull();
+  });
+
+  it('preserves a null parentPageId for a root page', () => {
+    const meta: DocmostMdMeta = {
+      version: 1,
+      pageId: 'root-1',
+      slugId: 'root-slug',
+      title: 'Root',
+      spaceId: 'space-1',
+      parentPageId: null,
+    };
+    const file = serializeDocmostMarkdownBody(meta, 'body text');
+    const parsed = parseDocmostMarkdown(file);
+    expect(parsed.meta).toEqual(meta);
+    expect(parsed.comments).toBeNull();
+  });
+
+  it('produces a parseable file for an empty/missing body', () => {
+    const meta: DocmostMdMeta = { version: 1, pageId: 'p-empty' };
+
+    // Empty string body.
+    const emptyFile = serializeDocmostMarkdownBody(meta, '');
+    expect(() => parseDocmostMarkdown(emptyFile)).not.toThrow();
+    const parsedEmpty = parseDocmostMarkdown(emptyFile);
+    expect(parsedEmpty.meta).toEqual(meta);
+    expect(parsedEmpty.body).toBe('');
+    expect(parsedEmpty.comments).toBeNull();
+
+    // Missing body (undefined) — serializer coalesces to "".
+    const missingFile = serializeDocmostMarkdownBody(
+      meta,
+      undefined as unknown as string,
+    );
+    expect(() => parseDocmostMarkdown(missingFile)).not.toThrow();
+    const parsedMissing = parseDocmostMarkdown(missingFile);
+    expect(parsedMissing.meta).toEqual(meta);
+    expect(parsedMissing.body).toBe('');
+    expect(parsedMissing.comments).toBeNull();
+  });
+});