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.d.ts

@@ -0,0 +1,13 @@
+/**
+ * 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 declare function bodyHash(markdownBody: string): string;