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/lib/diff.d.ts

@@ -0,0 +1,54 @@
+/**
+ * Headless, Docmost-equivalent document diff.
+ *
+ * Docmost's history editor computes a change set with the exact pipeline below
+ * (recreateTransform -> ChangeSet.addSteps -> simplifyChanges) and renders it as
+ * editor decorations. This module runs the SAME computation but serializes the
+ * result to text + integrity counts instead of decorations, so a diff can be
+ * previewed without a browser.
+ *
+ * recreateTransform here comes from @fellow/prosemirror-recreate-transform, the
+ * maintained published fork of the MIT prosemirror-recreate-steps source that
+ * Docmost vendors in @docmost/editor-ext; it exposes the identical
+ * recreateTransform(fromDoc, toDoc, { complexSteps, wordDiffs, simplifyDiff })
+ * signature.
+ *
+ * If recreateTransform / the changeset throws on a pathological document pair,
+ * we fall back to a coarse block-level text diff so the tool never hard-fails.
+ */
+/** A single inserted/deleted change with its containing-block context. */
+export interface DiffChange {
+    op: "insert" | "delete";
+    /** Lead (plain) text of the block that contains the change, for context. */
+    block: string;
+    /** The inserted or deleted text. */
+    text: string;
+}
+/** Integrity counts as [old, new] tuples; footnoteMarkers as [oldList, newList]. */
+export interface DiffIntegrity {
+    images: [number, number];
+    links: [number, number];
+    tables: [number, number];
+    callouts: [number, number];
+    footnoteMarkers: [number[], number[]];
+}
+export interface DiffResult {
+    summary: {
+        inserted: number;
+        deleted: number;
+        blocksChanged: number;
+    };
+    integrity: DiffIntegrity;
+    changes: DiffChange[];
+    /** Human-readable unified-ish summary. */
+    markdown: string;
+}
+/**
+ * Diff two ProseMirror JSON documents the way Docmost's history editor does and
+ * serialize the result to text + integrity counts.
+ *
+ * @param oldDocJson the earlier document
+ * @param newDocJson the later document
+ * @param notesHeading heading delimiting body from notes for footnote counting
+ */
+export declare function diffDocs(oldDocJson: any, newDocJson: any, notesHeading?: string): DiffResult;