Refine SKILL.md for airflow-translation agent skill

[sjyangkevin]

related: #61984

Why

Adds more clarity and make the instruction more like actionable steps. The previous instruction looks more like a document reference, and only a small section instructs the step-by-step workflow. However, the workflow can be different between adding a new translation and maintaining an existing translation. The updated instruction enhance the clarity, and provide step-by-step guideline on how to proceed with each task.

Added an example showing exactly what the scaffolded files will look like (e.g., "allRuns": "TODO: translate: All Runs"). This makes it more clear what the agent is supposed to look for and edit.

Moved reference material, such as the General Translation Rules, Terms Kept in English, and Plural Forms, below the workflow steps. Previously, an agent had to read through all the theoretical rules before figuring out what actually needed to be done. Now, the context is provided as reference material precisely when it's needed. Also, these rules may be override in locale-specific instruction; the phrasing also refined to make the instruction more clear when should follow the locale-specific guideline.

Added a validation step, to instruct running the completion check and prek command

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: [Antigravity] following the guidelines

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker f0a7828 v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.