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/vitest.config.ts

@@ -0,0 +1,23 @@
+import { fileURLToPath } from 'node:url';
+import path from 'node:path';
+import { defineConfig } from 'vitest/config';
+
+// Ported docmost-sync tests import the converter through the upstream package
+// barrel specifier `docmost-client`. We vendored only the PURE half of that
+// package into `src/lib`, so alias the barrel specifier to our local lib
+// barrel; everything those tests use (converter, canonicalize, markdown
+// envelope, markdownToProseMirror) is re-exported there.
+const here = path.dirname(fileURLToPath(import.meta.url));
+const libBarrel = path.resolve(here, 'src/lib/index.ts');
+
+export default defineConfig({
+  resolve: {
+    alias: {
+      'docmost-client': libBarrel,
+    },
+  },
+  test: {
+    environment: 'node',
+    include: ['test/**/*.test.ts'],
+  },
+});