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/src/engine/stabilize.ts

@@ -0,0 +1,58 @@
+/**
+ * Normalize-on-write helper (SPEC §11 "Резолюция").
+ *
+ * git diffs byte-for-byte, so writing a page in a NON-fixpoint markdown form
+ * would make the next pull re-export it to a slightly different (but stable)
+ * form and produce a phantom diff -> churny commits. The converter has a couple
+ * of known one-pass asymmetries (a block image after a paragraph adds an empty
+ * paragraph; a diagram materializes `data-align`), all of which converge to a
+ * fixpoint after ONE `export -> import -> export` round-trip.
+ *
+ * So at write time we run exactly that one pass and persist the fixpoint form.
+ * Already-stable content is unaffected (the pass is idempotent), so re-pulls of
+ * unchanged pages produce identical bytes and git sees no diff.
+ */
+import {
+  convertProseMirrorToMarkdown,
+  markdownToProseMirror,
+  serializeDocmostMarkdownBody,
+  type DocmostMdMeta,
+} from "../lib/index.js";
+
+/**
+ * Meta object as `exportPageBody` builds it (SPEC §4). Kept byte-for-byte
+ * compatible so files produced here match `exportPageBody`'s output exactly.
+ */
+export interface PageMeta {
+  version: 1;
+  pageId: string;
+  slugId: string;
+  title: string;
+  spaceId: string;
+  parentPageId: string | null;
+}
+
+/**
+ * Produce the self-contained `.md` file text for a page from its raw
+ * ProseMirror `content` + identity meta, in the verified fixpoint form.
+ *
+ *   md1        = convertProseMirrorToMarkdown(content)
+ *   doc2       = markdownToProseMirror(md1)            // one import...
+ *   stableBody = convertProseMirrorToMarkdown(doc2)    // ...and re-export
+ *   file       = serializeDocmostMarkdownBody(meta, stableBody)
+ *
+ * The single export->import->export pass is the verified fixpoint (SPEC §11):
+ * idempotent for already-stable content, and the convergence point for the
+ * known converter asymmetries.
+ */
+export async function stabilizePageFile(
+  content: unknown,
+  meta: PageMeta,
+): Promise<string> {
+  const md1 = convertProseMirrorToMarkdown(content);
+  const doc2 = await markdownToProseMirror(md1);
+  const stableBody = convertProseMirrorToMarkdown(doc2);
+  // The meta shape is exactly what `exportPageBody` writes; cast to the lib's
+  // DocmostMdMeta (a superset with optional fields) for the serializer.
+  return serializeDocmostMarkdownBody(meta as DocmostMdMeta, stableBody);
+}