Define Translation Agent Skills for Airflow Terminology

## Why

With recent agentic evolution, integrating LLM CLIs (e.g., Claude Code, Copilot CLI, Gemini CLI, etc.) with rendering prompts, such as in https://github.com/apache/airflow/pull/55917 for automating translations **can be replaced by [“Agent Skills”](https://agentskills.io) as a more generic, up-to-date, and lower-maintenance solution**.

## How

Add Airflow terminology–specific translation skills under `.github/skills/translation`.

- For `SKILL.md`, we should only mention **global-level** Airflow terminology guidelines.
- e.g., use `Dag` instead of `DAG`; some Airflow terms should remain in English, such as `Dag` and `XCom`.

- Each `locales/{locale-name}.md` file should document locale-specific guidelines, based on the existing locale files under `airflow-core/src/airflow/ui/public/i18n/locales/{locale-name}/`.

```bash
.github/skills/airflow-translations
├── locales
│ ├── ar.md
│ # ... all the locales we currently have, each with locale-specific skills
│ └── zh-TW.md
└── SKILL.md # global-level Airflow terminology guidelines
```

If your locale does not yet have ‎`airflow-core/src/airflow/ui/public/i18n/locales/{locale-name}/README.md`:

- Please describe why we selected specific terms for the local translation in that file.

## Sub-issues

- Global-level agent skills
- Locale-specific agent skills

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start by comparing the existing files under airflow-core/src/airflow/ui/public/i18n/locales/{locale-name}/, including any README.md files, with the requested .github/skills/translation structure. Define the global terminology in SKILL.md and document locale-specific choices in each locales/{locale-name}.md file, including reasons where a locale lacks a README.md.

Written by the indexing model from the issue text.