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/vault-registry.service.spec.ts

@@ -1,17 +1,30 @@
 // Unit tests for the per-space vault path resolver + lazy VaultGit cache
-// `mkdir` and `VaultGit` are mocked so construction is cheap and
+// `mkdir` and the git-sync loader are mocked so construction is cheap and
 // no real filesystem / git work happens. We assert the path normalization
 // (trailing slash) and the one-VaultGit-per-space caching contract.
+//
+// The service loads `VaultGit` (and `vaultGitEnv`) at runtime via the
+// `loadGitSync()` bridge (the ESM `@docmost/git-sync` package cannot be
+// `require()`d under jest), so we mock that loader rather than the package.
 import { mkdir } from 'node:fs/promises';
-import { VaultGit } from '@docmost/git-sync';
+import { loadGitSync } from '../git-sync.loader';
 
 jest.mock('node:fs/promises', () => ({
   mkdir: jest.fn(async () => undefined),
 }));
 
 // Cheap VaultGit stub: records the path it was constructed with; no shell-out.
-jest.mock('@docmost/git-sync', () => ({
-  VaultGit: jest.fn().mockImplementation((path: string) => ({ path })),
+// Declared with a `mock`-prefixed name so jest allows referencing it inside the
+// hoisted `jest.mock` factory below.
+const mockVaultGit = jest
+  .fn()
+  .mockImplementation((path: string) => ({ path }));
+
+jest.mock('../git-sync.loader', () => ({
+  loadGitSync: jest.fn(async () => ({
+    VaultGit: mockVaultGit,
+    vaultGitEnv: jest.fn(() => ({})),
+  })),
 }));
 
 import { VaultRegistryService } from './vault-registry.service';
@@ -19,7 +32,8 @@ import { VaultRegistryService } from './vault-registry.service';
 type AnyMock = jest.Mock;
 
 const mkdirMock = mkdir as unknown as AnyMock;
-const VaultGitMock = VaultGit as unknown as AnyMock;
+const VaultGitMock = mockVaultGit;
+void loadGitSync;
 
 function build(dataDir: string): { service: VaultRegistryService } {
   const env = {