Harden API endpoints: input validation, error sanitization, security headers

- Add shared validation helper (server/lib/validation.js) with ID regex, provider/type allowlists
- Validate all route params against allowlists before filesystem operations to prevent path traversal
- Strip stack traces and error.message from production error responses (generic "Internal server error")
- Sanitize filenames in Content-Disposition headers
- Add X-Content-Type-Options: nosniff and X-Frame-Options: DENY to dev server static responses
- Add path traversal (.. ) checks to all static file handlers and catch-all fetch

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

api/command-source/[id].js

@@ -6,9 +6,16 @@ const __filename = fileURLToPath(import.meta.url);
 const __dirname = dirname(__filename);
 const PROJECT_ROOT = join(__dirname, "../..");
 
+const VALID_ID = /^[a-zA-Z0-9_-]+$/;
+
 export default function handler(req, res) {
   try {
     const { id } = req.query;
+
+    if (!id || !VALID_ID.test(id)) {
+      return res.status(400).json({ error: "Invalid command ID" });
+    }
+
     const commandPath = join(PROJECT_ROOT, "source", "skills", id, "SKILL.md");
 
     if (!existsSync(commandPath)) {
@@ -19,6 +26,6 @@ export default function handler(req, res) {
     res.status(200).json({ content });
   } catch (error) {
     console.error("Error in /api/command-source:", error);
-    res.status(500).json({ error: error.message });
+    res.status(500).json({ error: "Internal server error" });
   }
 }