feat(git-sync): CommonJS build + §13.1 editor-ext idempotency gate (Phase A.2)

Make @docmost/git-sync natively consumable by the CommonJS server (and jest):
build to CommonJS (tsconfig module CommonJS, drop type:module, strip .js from
relative imports), and lazy-load the only ESM-only dep (marked) via the dynamic
Function('import()') trick (mirrors docmost-client.loader.ts) with a require()
fallback so vitest's evaluator works too. git-sync tests stay green (314 pass,
3 expected fail).

Add the §13.1 idempotency gate (apps/server .../git-sync-converter-gate.spec.ts):
13 editor-ext docs (paragraphs/headings, marks, links, bullet/ordered/task lists,
blockquote, callouts, code block, hr, table, nested mix) round-trip
content(editor-ext) -> convertProseMirrorToMarkdown -> markdownToProseMirror ->
TiptapTransformer.toYdoc/fromYdoc(tiptapExtensions) -> canonicalize and assert
docsCanonicallyEqual. All green => the vendored converter's docmost-schema is
schema-compatible with editor-ext (no node/mark/attr loss), which the plan §13.1
requires before Phase B. The one intrinsic markdown-image lossiness (width/height
/align can't ride plain ![](src)) is isolated in a KNOWN DIVERGENCE block, not
hidden. Server tsc clean.

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

packages/git-sync/package.json

@@ -3,11 +3,13 @@
   "version": "0.1.0",
   "description": "Vendored pure converter + pure sync engine for the Docmost <-> git Markdown sync (Phase A). See docs/git-sync-plan.md.",
   "private": true,
-  "type": "module",
   "main": "./build/index.js",
   "types": "./build/index.d.ts",
   "exports": {
-    ".": "./build/index.js"
+    ".": {
+      "types": "./build/index.d.ts",
+      "default": "./build/index.js"
+    }
   },
   "scripts": {
     "build": "tsc",