Add VERSION file as the single source for the extension version

Mirrors testium's src/VERSION: scripts/sync-version.js copies VERSION into
package.json#version (enforcing strict semver X.Y.Z, required by the marketplace
/ Open VSX). Wired into 'npm run package' and vscode:prepublish, so the version
is always synced before a build/publish. Bump = edit VERSION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/sync-version.js

@@ -0,0 +1,36 @@
+#!/usr/bin/env node
+/*
+ * Sync package.json "version" from the VERSION file — the single source of
+ * truth for the extension version (mirroring testium's src/VERSION).
+ *
+ * Bump the extension by editing VERSION, then run `npm run package` (or any
+ * flow that triggers vscode:prepublish): this script overwrites
+ * package.json#version to match. Unlike testium's VERSION (which may be X.Y),
+ * the marketplace / Open VSX require strict semver X.Y.Z, so VERSION must be
+ * X.Y.Z (optionally with a -prerelease suffix).
+ */
+const fs = require("fs");
+const path = require("path");
+
+const root = path.resolve(__dirname, "..");
+const versionFile = path.join(root, "VERSION");
+const pkgFile = path.join(root, "package.json");
+
+const version = fs.readFileSync(versionFile, "utf8").trim();
+if (!/^\d+\.\d+\.\d+(-[0-9A-Za-z.-]+)?$/.test(version)) {
+  console.error(
+    `sync-version: VERSION '${version}' is not strict semver (X.Y.Z) — ` +
+      `required by the marketplace / Open VSX.`,
+  );
+  process.exit(1);
+}
+
+const pkg = JSON.parse(fs.readFileSync(pkgFile, "utf8"));
+if (pkg.version === version) {
+  console.log(`sync-version: package.json already at ${version}`);
+} else {
+  const previous = pkg.version;
+  pkg.version = version;
+  fs.writeFileSync(pkgFile, JSON.stringify(pkg, null, 2) + "\n");
+  console.log(`sync-version: package.json ${previous} -> ${version}`);
+}