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.spec.ts

@@ -30,6 +30,21 @@ jest.mock('@hocuspocus/transformer', () => {
 jest.mock('@docmost/editor-ext', () => ({
   markdownToHtml: jest.fn(),
 }));
+// The service loads `parseDocmostMarkdown` / `markdownToProseMirror` at runtime
+// via the `loadGitSync()` bridge (the ESM `@docmost/git-sync` package cannot be
+// `require()`d under jest). Stub the loader: the real conversion is exercised by
+// the @docmost/git-sync converter tests and the converter gate; here the mocked
+// TiptapTransformer.toYdoc ignores the converted doc anyway, so a passthrough
+// body + a minimal ProseMirror doc is sufficient.
+jest.mock('../git-sync.loader', () => ({
+  loadGitSync: jest.fn(async () => ({
+    parseDocmostMarkdown: (md: string) => ({ meta: {}, body: md }),
+    markdownToProseMirror: async () => ({
+      type: 'doc',
+      content: [{ type: 'paragraph' }],
+    }),
+  })),
+}));
 
 import * as Y from 'yjs';
 import { GitmostDataSourceService } from './gitmost-datasource.service';