Add gitnexus-explorer optional skill (#5208)

Index codebases with GitNexus and serve an interactive knowledge
graph web UI via Cloudflare tunnel. No sudo required.

Includes:
- Full setup/build/serve/tunnel pipeline
- Zero-dependency Node.js reverse proxy script
- Pitfalls section covering cloudflared config conflicts,
  Vite allowedHosts, Claude Code artifact cleanup, and
  browser memory limits for large repos

optional-skills/research/gitnexus-explorer/scripts/proxy.mjs

@@ -0,0 +1,92 @@
+/**
+ * GitNexus reverse proxy — serves production web UI + proxies /api/* to backend.
+ * Zero dependencies, Node.js built-ins only.
+ *
+ * Usage: node proxy.mjs <dist-dir> [port]
+ *   dist-dir: path to gitnexus-web/dist (production build)
+ *   port: listen port (default: 8888)
+ *
+ * Environment:
+ *   API_PORT: GitNexus serve backend port (default: 4747)
+ */
+import http from 'node:http';
+import fs from 'node:fs';
+import path from 'node:path';
+
+const API_PORT = parseInt(process.env.API_PORT || '4747');
+const DIST_DIR = process.argv[2] || './dist';
+const PORT = parseInt(process.argv[3] || '8888');
+
+const MIME = {
+  '.html': 'text/html',
+  '.js': 'application/javascript',
+  '.css': 'text/css',
+  '.json': 'application/json',
+  '.png': 'image/png',
+  '.svg': 'image/svg+xml',
+  '.ico': 'image/x-icon',
+  '.woff2': 'font/woff2',
+  '.woff': 'font/woff',
+  '.wasm': 'application/wasm',
+  '.ttf': 'font/ttf',
+  '.map': 'application/json',
+};
+
+function proxyToApi(req, res) {
+  const opts = {
+    hostname: '127.0.0.1',
+    port: API_PORT,
+    path: req.url,
+    method: req.method,
+    headers: { ...req.headers, host: `127.0.0.1:${API_PORT}` },
+  };
+  const proxy = http.request(opts, (upstream) => {
+    res.writeHead(upstream.statusCode, upstream.headers);
+    upstream.pipe(res, { end: true });
+  });
+  proxy.on('error', () => {
+    res.writeHead(502, { 'Content-Type': 'text/plain' });
+    res.end('GitNexus backend unavailable — is `npx gitnexus serve` running?');
+  });
+  req.pipe(proxy, { end: true });
+}
+
+function serveStatic(req, res) {
+  const urlPath = req.url.split('?')[0];
+  let filePath = path.join(DIST_DIR, urlPath === '/' ? 'index.html' : urlPath);
+
+  // SPA fallback: if file doesn't exist and isn't a static asset, serve index.html
+  if (!fs.existsSync(filePath) && !path.extname(filePath)) {
+    filePath = path.join(DIST_DIR, 'index.html');
+  }
+
+  const ext = path.extname(filePath);
+  const mime = MIME[ext] || 'application/octet-stream';
+
+  try {
+    const data = fs.readFileSync(filePath);
+    res.writeHead(200, {
+      'Content-Type': mime,
+      'Cache-Control': ext === '.html' ? 'no-cache' : 'public, max-age=86400',
+    });
+    res.end(data);
+  } catch {
+    res.writeHead(404, { 'Content-Type': 'text/plain' });
+    res.end('Not found');
+  }
+}
+
+const server = http.createServer((req, res) => {
+  if (req.url.startsWith('/api')) {
+    proxyToApi(req, res);
+  } else {
+    serveStatic(req, res);
+  }
+});
+
+server.listen(PORT, () => {
+  console.log(`GitNexus proxy listening on http://localhost:${PORT}`);
+  console.log(`  Web UI: http://localhost:${PORT}/`);
+  console.log(`  API:    http://localhost:${PORT}/api/repos`);
+  console.log(`  Backend: http://127.0.0.1:${API_PORT}`);
+});