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
```

檢查所參照的 templates.hbs(github/workflows/automation) gist 以及 issue 中的安裝指令。新增 .github/workflows/auto-assign.yml、.github/workflows/dependabot-issue.yml 和 .github/dependabot.yml，並實作所描述的自動化。完成的標準是這三個 workflow 檔案都已存在，並針對列出的行為完成設定。

由索引模型根據 Issue 內容生成。