fix(readme): correct GitHub UTM attribution

[linqiang-max]

Summary

• Correct GitHub README links that were incorrectly attributed to Hugging Face
• Use utm_source=github and utm_medium=referral
• Add placement-specific utm_content values for header and Website Tools links
• Preserve provider pricing links and public API download links

Corrected attribution

• Header cost calculator
• Header model comparison
• Header budget planner
• Website Tools cost calculator
• Website Tools model comparison
• Website Tools budget planner

Validation

• No utm_source=huggingface remains in README
• No utm_medium=dataset remains in README
• Six placement-specific utm_content values are present
• Provider links remain unchanged
• Public JSON and CSV links remain unchanged
• README.md is the only changed file