docs: fix Claude Code plugin installation instructions

Author: 7nohe

.claude-plugin/marketplace.json

@@ -0,0 +1,18 @@
+{
+  "name": "7nohe/figram",
+  "owner": {
+    "name": "7nohe",
+    "url": "https://github.com/7nohe"
+  },
+  "plugins": [
+    {
+      "name": "figram",
+      "source": "..",
+      "description": "Claude Code skills for Figram - YAML-driven architecture diagrams for FigJam",
+      "version": "1.1.0",
+      "author": {
+        "name": "7nohe"
+      }
+    }
+  ]
+}

packages/docs/src/content/docs/en/installation.md

@@ -190,7 +190,11 @@ If you use [Claude Code](https://claude.ai/code), you can install the Figram plu
 ### Installation
 
 ```bash
-/plugin install 7nohe/figram
+# Add the marketplace
+/plugin marketplace add 7nohe/figram
+
+# Install the plugin
+/plugin install figram
 ```
 
 ### Available Skills

packages/docs/src/content/docs/ja/installation.md

@@ -190,7 +190,11 @@ npx figram serve diagram.yaml --icons path/to/my-icons.yaml
 ### インストール
 
 ```bash
-/plugin install 7nohe/figram
+# マーケットプレイスを追加
+/plugin marketplace add 7nohe/figram
+
+# プラグインをインストール
+/plugin install figram
 ```
 
 ### 利用可能なスキル