ci: 增加 Pull Request 的 Hugo 构建检查

[EthanH3514]

Summary

• add a read-only Hugo build check for pull requests targeting main
• use the same Hugo Extended 0.150.0 version as the production Pages workflow
• set up Go modules, cancel superseded runs, and keep PR checks separate from Pages and Algolia deployment

Verification

• parsed and validated the workflow YAML locally
• built the production site successfully with Hugo Extended 0.150.0 (696 pages, 732 HTML files)

Closes #20