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
```

Review the referenced templates.hbs(github/workflows/automation) gist and the installation command in the issue. Add .github/workflows/auto-assign.yml, .github/workflows/dependabot-issue.yml, and .github/dependabot.yml with the described automation. Done means the three workflow files are present and configured for the listed behaviors.

Vom Indexierungsmodell aus dem Issue-Text verfasst.