Add create-diff-issue job to update-system-prompt workflow

- Fetch old SYSTEM_PROMPT variable value before update
- Save old value to /tmp/old_value.txt for use in next job
- Add new create-diff-issue job that:
  - Reads old and new values
  - Creates diff between them
  - Creates issue with workflow and documentation labels
  - Issue title: [工作流] 提示词更新 <run_id>
  - Issue body includes workflow run link and diff

Co-Authored-By: Claude (mimo-v2-flash) <noreply@anthropic.com>

Author: WhiteElephantIsNotARobot

.github/workflows/update-system-prompt.yml

@@ -0,0 +1,83 @@
+name: Update System Prompt
+
+on:
+  push:
+    paths:
+      - 'system_prompt.md'
+  workflow_dispatch:
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    permissions:
+      actions: write
+      contents: read
+    env:
+      GH_TOKEN: ${{ secrets.REPOSITORY_VARIABLE_API_KEY }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 1
+
+      - name: Update SYSTEM_PROMPT variable
+        id: update
+        run: |
+          echo "正在更新 SYSTEM_PROMPT 变量..."
+
+          # 检查 system_prompt.md 文件是否存在
+          if [ ! -f "system_prompt.md" ]; then
+            echo "错误: system_prompt.md 文件不存在"
+            exit 1
+          fi
+
+          # 读取 system_prompt.md 内容
+          SYSTEM_PROMPT_CONTENT=$(cat system_prompt.md)
+
+          # 获取旧变量值
+          OLD_VALUE=$(gh api /repos/${{ github.repository }}/actions/variables/SYSTEM_PROMPT --jq '.value' 2>/dev/null || echo "")
+
+          # 更新 GitHub Actions 变量
+          gh api --method PATCH \
+            /repos/${{ github.repository }}/actions/variables/SYSTEM_PROMPT \
+            -f name=SYSTEM_PROMPT \
+            -f value="$SYSTEM_PROMPT_CONTENT"
+
+          echo "✓ SYSTEM_PROMPT 变量已更新"
+          echo "文件大小: $(wc -c < system_prompt.md) 字符"
+
+          # 保存旧值到文件供下一步使用
+          echo "$OLD_VALUE" > /tmp/old_value.txt
+
+  create-diff-issue:
+    needs: update
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      contents: read
+    env:
+      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+      - name: Create diff issue
+        run: |
+          # 读取旧值
+          OLD_VALUE=$(cat /tmp/old_value.txt)
+          # 读取新值
+          NEW_VALUE=$(cat system_prompt.md)
+
+          # 构建 diff 内容
+          DIFF_CONTENT=$(diff -u <(echo "$OLD_VALUE") <(echo "$NEW_VALUE") 2>/dev/null || echo "No diff available")
+
+          # 构建 issue 内容
+          ISSUE_BODY="[工作流运行 #${{ github.run_id }}](https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }})
+
+\`\`\`diff
+${DIFF_CONTENT}
+\`\`\`"
+
+          # 创建 issue
+          gh issue create \
+            --title "[工作流] 提示词更新 ${{ github.run_id }}" \
+            --body "$ISSUE_BODY" \
+            --label workflow \
+            --label documentation