Add devcontainer (#274)

* feat: add Dockerfile and devcontainer configuration for Node.js development

* feat: add devcontainer features for Node.js and Claude integration

* Update .devcontainer/Dockerfile

Author: kentaro-m

.devcontainer/Dockerfile

@@ -0,0 +1,2 @@
+FROM mcr.microsoft.com/vscode/devcontainers/javascript-node:20
+

.devcontainer/devcontainer.json

@@ -0,0 +1,24 @@
+{
+  "name": "Node.js DevContainer",
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
+  "features": {
+    "ghcr.io/devcontainers/features/node:1": {},
+    "ghcr.io/anthropics/devcontainer-features/claude-code:1": {}
+  },
+  "customizations": {
+    "vscode": {
+      "settings": {
+        "terminal.integrated.shell.linux": "/bin/bash"
+      },
+      "extensions": [
+        "dbaeumer.vscode-eslint",
+        "esbenp.prettier-vscode"
+      ]
+    }
+  },
+  "postCreateCommand": "npm install",
+  "forwardPorts": [],
+  "remoteUser": "node"
+}