Add script to clean materialized plugin files

Adds eng/clean-materialized-plugins.mjs which removes agents/, commands/,
and skills/ subdirectories from plugin folders. These are generated by CI
via eng/materialize-plugins.mjs during publish and should not be tracked on
the staged branch.

Runnable via: npm run plugin:clean

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

eng/clean-materialized-plugins.mjs

@@ -0,0 +1,62 @@
+#!/usr/bin/env node
+
+import fs from "fs";
+import path from "path";
+import { ROOT_FOLDER } from "./constants.mjs";
+
+const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
+const MATERIALIZED_DIRS = ["agents", "commands", "skills"];
+
+function cleanPlugin(pluginPath) {
+  let removed = 0;
+  for (const subdir of MATERIALIZED_DIRS) {
+    const target = path.join(pluginPath, subdir);
+    if (fs.existsSync(target) && fs.statSync(target).isDirectory()) {
+      const count = countFiles(target);
+      fs.rmSync(target, { recursive: true, force: true });
+      removed += count;
+      console.log(`  Removed ${path.basename(pluginPath)}/${subdir}/ (${count} files)`);
+    }
+  }
+  return removed;
+}
+
+function countFiles(dir) {
+  let count = 0;
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    if (entry.isDirectory()) {
+      count += countFiles(path.join(dir, entry.name));
+    } else {
+      count++;
+    }
+  }
+  return count;
+}
+
+function main() {
+  console.log("Cleaning materialized files from plugins...\n");
+
+  if (!fs.existsSync(PLUGINS_DIR)) {
+    console.error(`Error: plugins directory not found at ${PLUGINS_DIR}`);
+    process.exit(1);
+  }
+
+  const pluginDirs = fs.readdirSync(PLUGINS_DIR, { withFileTypes: true })
+    .filter(entry => entry.isDirectory())
+    .map(entry => entry.name)
+    .sort();
+
+  let total = 0;
+  for (const dirName of pluginDirs) {
+    total += cleanPlugin(path.join(PLUGINS_DIR, dirName));
+  }
+
+  console.log();
+  if (total === 0) {
+    console.log("✅ No materialized files found. Plugins are already clean.");
+  } else {
+    console.log(`✅ Removed ${total} materialized file(s) from plugins.`);
+  }
+}
+
+main();