feat: add hello-world MCP server scaffold

Minimal dependency-free Node.js MCP server (JSON-RPC 2.0 over stdio).
Exposes one tool: say_hello. Handles initialize, tools/list, tools/call,
and SIGTERM cleanly. No npm packages required — uses only readline.

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

plugins/hello-world/bin/mcp-server.js

@@ -0,0 +1,69 @@
+#!/usr/bin/env node
+'use strict';
+
+// Minimal MCP server — JSON-RPC 2.0 over stdio, no external dependencies.
+// Exposes one tool: `say_hello`. Replace with your actual tools.
+
+const readline = require('readline');
+
+const rl = readline.createInterface({ input: process.stdin, terminal: false });
+
+function send(obj) {
+  process.stdout.write(JSON.stringify(obj) + '\n');
+}
+
+const TOOLS = [
+  {
+    name: 'say_hello',
+    description: 'Say hello from the holocron marketplace hello-world plugin.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        name: { type: 'string', description: 'Name to greet (optional)' }
+      }
+    }
+  }
+];
+
+rl.on('line', (line) => {
+  let msg;
+  try { msg = JSON.parse(line); } catch { return; }
+
+  const { method, id, params } = msg;
+
+  if (method === 'initialize') {
+    send({
+      jsonrpc: '2.0', id,
+      result: {
+        protocolVersion: '2024-11-05',
+        capabilities: { tools: {} },
+        serverInfo: { name: 'hello-world', version: '1.0.0' }
+      }
+    });
+
+  } else if (method === 'notifications/initialized') {
+    // no response required
+
+  } else if (method === 'tools/list') {
+    send({ jsonrpc: '2.0', id, result: { tools: TOOLS } });
+
+  } else if (method === 'tools/call') {
+    const toolName = params?.name;
+    const args = params?.arguments ?? {};
+
+    if (toolName === 'say_hello') {
+      const greeting = args.name
+        ? `Hello, ${args.name}! The holocron marketplace hello-world plugin is working.`
+        : 'Hello from the holocron marketplace hello-world plugin!';
+      send({ jsonrpc: '2.0', id, result: { content: [{ type: 'text', text: greeting }] } });
+    } else {
+      send({ jsonrpc: '2.0', id, error: { code: -32601, message: `Unknown tool: ${toolName}` } });
+    }
+
+  } else if (id !== undefined) {
+    send({ jsonrpc: '2.0', id, error: { code: -32601, message: 'Method not found' } });
+  }
+});
+
+process.on('SIGINT', () => process.exit(0));
+process.on('SIGTERM', () => process.exit(0));