feat(skills): defer asset loading until the section is expanded

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Summary

• Collapse the skill asset files section by default, and expand it only when the user opens it.
• Skip draft and published asset fetches until the first expansion, so skill detail pages do not load assets on every visit.
• After the first open, collapsing the section keeps uploads and inputs in place.

Test plan

• Open a skill detail page and confirm the asset files section is collapsed and no draft/published asset requests fire.
• Expand the section and confirm the asset list, quota, and upload controls load.
• Collapse and expand again and confirm staged uploads and inputs are preserved.
• Confirm export, preview, and delete still work after the section is expanded.

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