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/sanitize.ts

@@ -0,0 +1,109 @@
+/**
+ * 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.
+ */
+
+// Printable characters forbidden in file names on common filesystems (mainly
+// Windows): / \ < > : " | ? *. Each match is replaced with a single "-".
+// Spaces are NOT in this set; whitespace is normalized separately below.
+// ASCII control characters (code points 0..31) are stripped in a separate pass
+// (see stripControlChars) to keep this literal free of embedded control bytes.
+const FORBIDDEN_PRINTABLE_RE = /[/\\<>:"|?*]/g;
+
+// Runs of whitespace (including tabs/newlines) collapse to a single space.
+const WHITESPACE_RUN_RE = /\s+/g;
+
+// Reserved Windows device names (case-insensitive). A bare match (with or
+// without an extension) is unusable as a file name, so it is prefixed with "_".
+const RESERVED_WINDOWS_NAMES = new Set([
+  "con",
+  "prn",
+  "aux",
+  "nul",
+  "com1",
+  "com2",
+  "com3",
+  "com4",
+  "com5",
+  "com6",
+  "com7",
+  "com8",
+  "com9",
+  "lpt1",
+  "lpt2",
+  "lpt3",
+  "lpt4",
+  "lpt5",
+  "lpt6",
+  "lpt7",
+  "lpt8",
+  "lpt9",
+]);
+
+// Cap on the sanitized length to stay well within filesystem path-component
+// limits (255 bytes on most FSes) while leaving room for an extension and a
+// disambiguation suffix.
+const MAX_LENGTH = 120;
+
+/**
+ * Replace every ASCII control character (code points 0..31) with "-". Done by
+ * scanning code points rather than a control-range regex literal, so the source
+ * file carries no embedded control bytes.
+ */
+function stripControlChars(input: string): string {
+  let out = "";
+  for (let i = 0; i < input.length; i++) {
+    out += input.charCodeAt(i) < 32 ? "-" : input[i];
+  }
+  return out;
+}
+
+/**
+ * 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 function sanitizeTitle(title: string): string {
+  let name = stripControlChars(title ?? "")
+    .replace(FORBIDDEN_PRINTABLE_RE, "-")
+    .replace(WHITESPACE_RUN_RE, " ")
+    .trim();
+
+  if (name.length > MAX_LENGTH) {
+    name = name.slice(0, MAX_LENGTH).trim();
+  }
+
+  // Compare the base name (before the first dot) against reserved names, so
+  // both "CON" and "con.md" are caught.
+  const base = name.split(".")[0]?.toLowerCase() ?? "";
+  // A name that is empty, consists only of dots ("." / ".." / "..."), or is a
+  // reserved Windows device name is unusable as a path component. The all-dots
+  // case is a path-traversal hazard in particular: an unprefixed ".." would
+  // become a parent-directory segment and let a page escape the vault, so it
+  // MUST be neutralized here (becomes "_..", which is a literal file name).
+  if (
+    name.length === 0 ||
+    /^\.+$/.test(name) ||
+    RESERVED_WINDOWS_NAMES.has(base)
+  ) {
+    name = "_" + name;
+  }
+
+  return name;
+}
+
+/**
+ * Disambiguate a sanitized name when two siblings in the same folder collapse
+ * to the same name. Appends a stable suffix built from the page's `slugId`, so
+ * the result stays deterministic across runs (SPEC §12: `Title ~slugId`).
+ */
+export function disambiguate(name: string, slugId: string): string {
+  return `${name} ~${slugId}`;
+}