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/sanitize.d.ts

@@ -0,0 +1,23 @@
+/**
+ * Deterministic filename strategy (SPEC §12).
+ *
+ * The file name is COSMETIC — the source of truth for the file<->page link is
+ * `pageId` / `slugId` inside the meta block, so renaming a file is safe. These
+ * functions are intentionally dependency-free and pure, so they are trivially
+ * unit-testable.
+ */
+/**
+ * Sanitize a page title into a safe file-name component (WITHOUT extension).
+ *
+ * Steps: replace forbidden / control characters with "-", collapse whitespace
+ * runs to a single space, trim, cap the length, then guard against an empty
+ * result, an all-dots result, or a reserved Windows device name by prefixing
+ * with "_".
+ */
+export declare function sanitizeTitle(title: string): string;
+/**
+ * Disambiguate a sanitized name when two siblings in the same folder collapse
+ * to the same name. Appends a stable suffix built from the page's `slugId`, so
+ * the result stays deterministic across runs (SPEC §12: `Title ~slugId`).
+ */
+export declare function disambiguate(name: string, slugId: string): string;