feat: add generic mcp-bridge for proxying upstream MCP SSE servers

- Add mcp-bridge/index.js: WebSocket backend for hub, relays requests
  to upstream MCP SSE servers via SSE+POST transport
- Add mcp-bridge/package.json with 'ws' dependency
- Add 'mcp-bridge-memory' entry to ecosystem.config.js (not started)
- Add 'bridge' script to root package.json
- Add mcp-bridge/node_modules to .gitignore

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

package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "test": "node test/e2e.js",
     "start": "node src/index.js",
+    "bridge": "node mcp-bridge/index.js",
     "sample-mcp": "node sample-mcp/index.js",
     "pm2:start": "pm2 start ecosystem.config.js",
     "pm2:stop": "pm2 stop all",