feat(git-sync): vendor pure converter + engine into @docmost/git-sync (Phase A.1)

First step of docs/git-sync-plan.md. New workspace package @docmost/git-sync
vendoring the PURE parts from docmost-sync (HEAD b03eb35):
- lib: markdown-converter, markdown-document, canonicalize, docmost-schema,
  node-ops, diff, and an extracted markdown-to-prosemirror (only the pure
  marked->HTML->generateJSON path from upstream collaboration.ts; no websocket).
- engine (pure, no IO): reconcile, layout, sanitize, stabilize, loop-guard.
Ported the upstream pure-module + round-trip corpus tests (vitest): 314 pass,
3 expected upstream known-limitation fails. tsc clean. No server wiring yet.

docmost-schema inlines getStyleProperty (as packages/mcp does — @tiptap/core
3.20.4 doesn't export it). IO engine (pull/push/git/settings) deferred to later
Phase A/B steps; the editor-ext idempotency gate (plan §13.1) is the next step.

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

packages/git-sync/package.json

@@ -0,0 +1,41 @@
+{
+  "name": "@docmost/git-sync",
+  "version": "0.1.0",
+  "description": "Vendored pure converter + pure sync engine for the Docmost <-> git Markdown sync (Phase A). See docs/git-sync-plan.md.",
+  "private": true,
+  "type": "module",
+  "main": "./build/index.js",
+  "types": "./build/index.d.ts",
+  "exports": {
+    ".": "./build/index.js"
+  },
+  "scripts": {
+    "build": "tsc",
+    "watch": "tsc --watch",
+    "test": "vitest run",
+    "test:watch": "vitest"
+  },
+  "license": "MIT",
+  "dependencies": {
+    "@fellow/prosemirror-recreate-transform": "^1.2.3",
+    "@tiptap/core": "3.20.4",
+    "@tiptap/extension-highlight": "3.20.4",
+    "@tiptap/extension-image": "3.20.4",
+    "@tiptap/extension-subscript": "3.20.4",
+    "@tiptap/extension-superscript": "3.20.4",
+    "@tiptap/extension-task-item": "3.20.4",
+    "@tiptap/extension-task-list": "3.20.4",
+    "@tiptap/html": "3.20.4",
+    "@tiptap/pm": "3.20.4",
+    "@tiptap/starter-kit": "3.20.4",
+    "jsdom": "25.0.0",
+    "marked": "17.0.5"
+  },
+  "devDependencies": {
+    "@types/jsdom": "^21.1.7",
+    "@types/node": "^20.0.0",
+    "fast-check": "^4.8.0",
+    "typescript": "^5.0.0",
+    "vitest": "4.1.6"
+  }
+}