docs: replace canonical header image

[LightDevCoder]

Summary

• replace the canonical README header raster at skills/docs/assets/skills-header.png with the supplied 1536x1024 image
• update the asset manifest and header validation to record the new PNG dimensions and SHA-256
• retain the editable skills-header.svg source and preserve the canonical asset path in both README files

Validation

• HEADER_ASSETS=PASS (11 assertions)
• COLLECTION_DISCOVERY=PASS (689 assertions)
• QUICK_START=PASS (8 assertions)
• supplied-image and repository PNG SHA-256 match
• no temporary versioned image remains