refactor(git-sync): consume @docmost/prosemirror-markdown, drop the duplicate lib (#293 stage 3 / no-op)

git-sync's converter-core (src/lib) was a byte-identical duplicate of the new
@docmost/prosemirror-markdown package (created in the previous commit). Switch
git-sync to consume the package and delete its copy — ending the duplication
that the whole #293 effort targets. Pure no-op: NO format/behavior change.

- git-sync depends on @docmost/prosemirror-markdown (workspace:*); engine
  (stabilize/push/pull) + src/index barrel + 12 engine tests re-point their
  converter imports to the package.
- Delete git-sync/src/lib (8 files) and the 23 duplicate converter-core test
  files + their fixtures — the converter and its ~440 tests now live once, in the
  package. git-sync keeps only its ENGINE tests, which exercise the converter
  through the package (the no-op proof). Kept roundtrip-helpers.ts (an engine
  test imports firstDivergence from it; pure helper, no double-run).
- Added docmostExtensions to the package barrel (a kept engine schema-validity
  test needs it).

Verified: editor-ext + prosemirror-markdown + git-sync all tsc EXIT 0;
git-sync vitest 28 files, 268 passed, 0 failures (engine cycle/roundtrip/push/
pull/reconcile green = no-op proof); prosemirror-markdown vitest still 443 passed
| 1 expected-fail; pnpm --frozen-lockfile EXIT 0; no ../lib refs remain in git-sync.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/git-sync/src/engine/pull.ts

@@ -31,7 +31,7 @@
  */
 import { dirname } from "node:path";
 import { sep } from "node:path";
-import { parsePageFile, serializePageFile } from "../lib/page-file.js";
+import { parsePageFile, serializePageFile } from "@docmost/prosemirror-markdown";
 import type { GitSyncClient } from "./client.types.js";
 import { buildVaultLayout, type PageNode } from "./layout.js";
 import {