feat: restore the asset-transformer-toolkit skill for toolkit 4.0

[ziyiunity]

What

Restores the asset-transformer-toolkit skill, which was pulled before launch because the API it relies on (ATTAssistantUtilities) wasn't available outside the AI Assistant package. Asset Transformer Toolkit 4.0.0 is now released and that API ships in a regular assembly, so the skill works with any coding agent.

Changes

• Skill content is the same as the pre-launch version, already adapted for coding agents (plain C# run in a live Editor rather than an Assistant-specific command wrapper).
• New "Before you start" section: the agent reads the resolved com.unity.industry.toolkit version first. Not installed: explain it's needed and stop, without adding it to the manifest. Below 4.0.0: report the version and ask before upgrading, with no fallback to older APIs or reflection. required_packages stays in the frontmatter, but agents don't enforce it, so the check has to be in the body.
• Description shortened to 248 characters, in line with the description budget work in docs(skills): shorten and front-load the skill descriptions #75.

Verify

npm install --no-save js-yaml@4
node scripts/check-skill-frontmatter.mjs skills