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/build/engine/sanitize.js

@@ -1,3 +1,4 @@
+"use strict";
 /**
  * Deterministic filename strategy (SPEC §12).
  *
@@ -6,6 +7,9 @@
  * functions are intentionally dependency-free and pure, so they are trivially
  * unit-testable.
  */
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.sanitizeTitle = sanitizeTitle;
+exports.disambiguate = disambiguate;
 // Printable characters forbidden in file names on common filesystems (mainly
 // Windows): / \ < > : " | ? *. Each match is replaced with a single "-".
 // Spaces are NOT in this set; whitespace is normalized separately below.
@@ -64,7 +68,7 @@ function stripControlChars(input) {
  * result, an all-dots result, or a reserved Windows device name by prefixing
  * with "_".
  */
-export function sanitizeTitle(title) {
+function sanitizeTitle(title) {
     let name = stripControlChars(title ?? "")
         .replace(FORBIDDEN_PRINTABLE_RE, "-")
         .replace(WHITESPACE_RUN_RE, " ")
@@ -92,6 +96,6 @@ export function sanitizeTitle(title) {
  * to the same name. Appends a stable suffix built from the page's `slugId`, so
  * the result stays deterministic across runs (SPEC §12: `Title ~slugId`).
  */
-export function disambiguate(name, slugId) {
+function disambiguate(name, slugId) {
     return `${name} ~${slugId}`;
 }