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/node_modules/.bin/yaml

@@ -0,0 +1,17 @@
+#!/bin/sh
+basedir=$(dirname "$(echo "$0" | sed -e 's,\\,/,g')")
+
+case `uname` in
+    *CYGWIN*) basedir=`cygpath -w "$basedir"`;;
+esac
+
+if [ -z "$NODE_PATH" ]; then
+  export NODE_PATH="/home/claude/gitmost/node_modules/.pnpm/yaml@2.8.3/node_modules/yaml/node_modules:/home/claude/gitmost/node_modules/.pnpm/yaml@2.8.3/node_modules:/home/claude/gitmost/node_modules/.pnpm/node_modules"
+else
+  export NODE_PATH="/home/claude/gitmost/node_modules/.pnpm/yaml@2.8.3/node_modules/yaml/node_modules:/home/claude/gitmost/node_modules/.pnpm/yaml@2.8.3/node_modules:/home/claude/gitmost/node_modules/.pnpm/node_modules:$NODE_PATH"
+fi
+if [ -x "$basedir/node" ]; then
+  exec "$basedir/node"  "$basedir/../../../../node_modules/.pnpm/yaml@2.8.3/node_modules/yaml/bin.mjs" "$@"
+else
+  exec node  "$basedir/../../../../node_modules/.pnpm/yaml@2.8.3/node_modules/yaml/bin.mjs" "$@"
+fi