feat: Add automation workflows (auto-assign, dependabot)

## Description

Add automation workflows from `templates.hbs(github/workflows/automation)` gist:

## Files

- `.github/workflows/auto-assign.yml` - Auto-assigns owner to issues/PRs (single maintainer repos)
- `.github/workflows/dependabot-issue.yml` - Creates tracking issues for Dependabot PRs
- `.github/dependabot.yml` - Weekly GitHub Actions version updates

## Source

Installed via:
```bash
gh gist clone eb3e040449ade0b0b0611b93cea454f2 gist-github-workflows-auto
INSTALL_DIR=. just --justfile gist-github-workflows-auto/github_workflows_auto.just install
```

从所引用的自动化 gist 开始，并运行所示的 `gh gist clone` 和 `just` 安装命令。检查指定的三个文件：`.github/workflows/auto-assign.yml`、`.github/workflows/dependabot-issue.yml` 和 `.github/dependabot.yml`；完成的标准是所请求的工作流和每周的 GitHub Actions 更新都已存在。

由索引模型根据 Issue 内容生成。