fix: takt-review を pull_request_target に変更して fork PR でもシークレットを利用可能に

- pull_request → pull_request_target でベースリポジトリのコンテキストで実行
- fork リポジトリからのチェックアウトに repository を追加
- ANTHROPIC_API_KEY が空の場合に即座にエラー終了するガードを追加

.github/workflows/takt-review.yml

@@ -1,7 +1,7 @@
 name: TAKT PR Review
 
 on:
-  pull_request:
+  pull_request_target:
     types: [opened, synchronize, ready_for_review, reopened]
 
 jobs:
@@ -15,8 +15,18 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: ${{ github.event.pull_request.head.sha }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
           fetch-depth: 0
 
+      - name: API キー確認
+        run: |
+          if [ -z "$ANTHROPIC_API_KEY" ]; then
+            echo "::error::ANTHROPIC_API_KEY is not set"
+            exit 1
+          fi
+        env:
+          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
+
       - uses: actions/setup-node@v4
         with:
           node-version: 20