Add missing /api/command-source Vercel endpoint

This endpoint was in the local server but missing from Vercel serverless
functions, causing "Source not available" on the deployed site.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

api/command-source/[id].js

@@ -0,0 +1,25 @@
+import { readFileSync, existsSync } from "fs";
+import { join, dirname } from "path";
+import { fileURLToPath } from "url";
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+const PROJECT_ROOT = join(__dirname, "../..");
+
+export default function handler(req, res) {
+  try {
+    const { id } = req.query;
+    const sourceDir = join(PROJECT_ROOT, "source");
+    const commandPath = join(sourceDir, "commands", `${id}.md`);
+
+    if (!existsSync(commandPath)) {
+      return res.status(404).json({ error: "Command not found" });
+    }
+
+    const content = readFileSync(commandPath, "utf-8");
+    res.status(200).json({ content });
+  } catch (error) {
+    console.error("Error in /api/command-source:", error);
+    res.status(500).json({ error: error.message });
+  }
+}