✨ feat: add pull request template and enforce branching strategy in workflow

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+### PR 类型
+
+- [ ] Bug 修复
+- [ ] 新功能
+- [ ] 文档更新
+- [ ] 其他
+
+### PR 是否包含破坏性更新？
+
+- [ ] 是
+- [ ] 否
+
+### PR 描述
+
+**请在下方详细描述您的 PR，包括目的、实现细节等。**
+
+### **重要提示**
+
+**所有 PR 都必须提交到 `alpha` 分支。请确保您的 PR 目标分支是 `alpha`。**

.github/workflows/pr-target-branch-check.yml

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+name: Check PR Branching Strategy
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, edited]
+
+jobs:
+  check-branching-strategy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Enforce branching strategy
+        run: |
+          if [[ "${{ github.base_ref }}" == "main" ]]; then
+            if [[ "${{ github.head_ref }}" != "alpha" ]]; then
+              echo "Error: Pull requests to 'main' are only allowed from the 'alpha' branch."
+              exit 1
+            fi
+          elif [[ "${{ github.base_ref }}" != "alpha" ]]; then
+            echo "Error: Pull requests must be targeted to the 'alpha' or 'main' branch."
+            exit 1
+          fi
+          echo "Branching strategy check passed."