refactor(git-sync): internalize the engine — first-class ESM, no vendoring bridge (#119 review)

Closes the architecture item from the #119 review: drop the "vendored from
docmost-sync" framing and the CJS↔ESM `Function('import()')` bridge so the engine
is a normal first-class gitmost package.

Part 1 — vendoring markers removed (prose only, zero behavior change): reworded
"VENDORED into gitmost" / "vendored from docmost-sync" / "Engine LOGIC is
byte-identical" / "it's a port" comments across the engine. Behavior-bearing
strings are untouched: BOT_AUTHOR_NAME/EMAIL and the `Docmost-Sync-Source:`
provenance trailers (changing them would break git authorship + the loop-guard).

Part 2 — the package is now ESM (matching the sibling @docmost/mcp): `type: module`,
tsconfig Node16, `.js` extensions on relative imports, and a static
`import { marked }` replacing the `new Function('return import(...)')` /
`loadMarked` hack — the bridge is GONE from the package. The CommonJS NestJS
server loads the now-ESM engine via a new `git-sync.loader.ts` that mirrors the
existing `docmost-client.loader.ts` mcp loader exactly (Function-indirected
dynamic import + cached promise + retry-on-reject). The 4 server consumers
(orchestrator/datasource/vault-registry/git-http-backend) call `await loadGitSync()`
for value exports; types stay `import type` (erased). The converter-gate spec —
which needs the real converter — loads the package's TS source via a jest
moduleNameMapper + isolatedModules (documented in that spec); the other git-sync
specs mock the loader.

Verified: engine builds pure ESM (no Function/require leftover), vitest 614,
editor-ext build, server + client tsc, full server jest 1397/0. Live stand
smoke-test: server starts clean on the ESM engine (no ERR_REQUIRE_ESM), a real
sync cycle runs through the loader, and the basic e2e suite is 12/12 (clone via
git-http-backend, push, pull, delete, 3-way merge — all through the new loader).

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

apps/server/src/integrations/git-sync/services/gitmost-datasource.service.ts

@@ -1,12 +1,11 @@
 import { Injectable, Logger, NotFoundException } from '@nestjs/common';
 import { TiptapTransformer } from '@hocuspocus/transformer';
 import { generateJitteredKeyBetween } from 'fractional-indexing-jittered';
-import {
-  type GitSyncClient,
-  type GitSyncPageNodeLite,
-  parseDocmostMarkdown,
-  markdownToProseMirror,
+import type {
+  GitSyncClient,
+  GitSyncPageNodeLite,
 } from '@docmost/git-sync';
+import { loadGitSync } from '../git-sync.loader';
 import { PageRepo } from '@docmost/db/repos/page/page.repo';
 import { SpaceRepo } from '@docmost/db/repos/space/space.repo';
 import { InjectKysely } from 'nestjs-kysely';
@@ -177,6 +176,7 @@ export class GitmostDataSourceService {
     fullMarkdown: string,
     baseMarkdown?: string | null,
   ): Promise<{ updatedAt?: string }> {
+    const { parseDocmostMarkdown, markdownToProseMirror } = await loadGitSync();
     const { body } = parseDocmostMarkdown(fullMarkdown);
     const doc = await markdownToProseMirror(body);
 
@@ -213,6 +213,7 @@ export class GitmostDataSourceService {
     );
 
     // The shell is created without body; push the markdown body through collab.
+    const { parseDocmostMarkdown, markdownToProseMirror } = await loadGitSync();
     const { body } = parseDocmostMarkdown(content);
     const doc = await markdownToProseMirror(body);
     await this.writeBody(page.id, doc, ctx.userId);