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/build/engine/loop-guard.js

@@ -0,0 +1,28 @@
+/**
+ * Loop-guard primitives (SPEC §10). The sync engine must never re-pull its OWN
+ * write as if it were a remote edit: after a push, the next poll will see the
+ * page it just wrote with a fresh `updatedAt`. To suppress that, we key on two
+ * signals — the body HASH of what we pushed (this module) and the `updatedAt`
+ * returned by the write — recorded per page at push time.
+ *
+ * This module owns the PURE, deterministic body-hash. The CONSUMPTION on the
+ * pull side (comparing an incoming page's body hash against the last pushed hash
+ * to decide "this is our own write, ignore it") is a future increment — here we
+ * only PRODUCE the hash and the per-page push record (see `src/push.ts`).
+ */
+import { createHash } from "node:crypto";
+/**
+ * Stable hash of a page's markdown BODY (SPEC §10 "хэш тела"). Deterministic:
+ * the same input string always yields the same digest, a different input a
+ * different one. Used to recognize our own write later (loop suppression).
+ *
+ * We hash the body STRING as-is (UTF-8) with SHA-256 and return lowercase hex.
+ * SPEC §10 keys on the body hash rather than file bytes; callers decide WHAT
+ * counts as "the body" (here it is the exact string passed in — typically the
+ * self-contained markdown that was pushed). No normalization is applied: the
+ * caller is responsible for passing a canonical/stable representation if it
+ * wants hash equality across cosmetic-only differences.
+ */
+export function bodyHash(markdownBody) {
+    return createHash("sha256").update(markdownBody, "utf8").digest("hex");
+}