Add CI workflow to validate marketplace.json on PRs

Add a GitHub Actions workflow that validates marketplace.json is
well-formed JSON with a plugins array whenever PRs modify it. Includes:
- validate-marketplace.ts: Bun script that parses and validates the JSON
- validate-marketplace.yml: GH Actions workflow triggered on PR changes
- test-marketplace-check.js: Unit tests for the validation logic

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/scripts/validate-marketplace.ts

@@ -0,0 +1,49 @@
+#!/usr/bin/env bun
+/**
+ * Validates that marketplace.json is well-formed JSON with a plugins array.
+ *
+ * Usage:
+ *   bun validate-marketplace.ts <path-to-marketplace.json>
+ */
+
+import { readFile } from "fs/promises";
+
+async function main() {
+  const filePath = process.argv[2];
+  if (!filePath) {
+    console.error("Usage: validate-marketplace.ts <path-to-marketplace.json>");
+    process.exit(2);
+  }
+
+  const content = await readFile(filePath, "utf-8");
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(content);
+  } catch (err) {
+    console.error(
+      `ERROR: ${filePath} is not valid JSON: ${err instanceof Error ? err.message : err}`
+    );
+    process.exit(1);
+  }
+
+  if (!parsed || typeof parsed !== "object" || Array.isArray(parsed)) {
+    console.error(`ERROR: ${filePath} must be a JSON object`);
+    process.exit(1);
+  }
+
+  const marketplace = parsed as Record<string, unknown>;
+  if (!Array.isArray(marketplace.plugins)) {
+    console.error(`ERROR: ${filePath} missing "plugins" array`);
+    process.exit(1);
+  }
+
+  console.log(
+    `marketplace.json is valid (${marketplace.plugins.length} plugins)`
+  );
+}
+
+main().catch((err) => {
+  console.error("Fatal error:", err);
+  process.exit(2);
+});