[codex] localize guide page styles

[Sean-Kenneth-Doherty]

Summary

• remove the guide page dependency on the external unpkg/chota stylesheet
• add a small local style block for readable article typography and layout
• avoid one third-party CSS request on the open-source guide page

Addresses #470

Validation

• git diff --check
• source check confirms src/pages/contribute-to-opensource.astro no longer references unpkg.com/chota
• GITHUB_TOKEN=$(gh auth token) pnpm build
• generated dist/contribute-to-opensource/index.html contains no unpkg or chota.min.css reference
• generated guide HTML still contains the guide title and local container styling

[Sean-Kenneth-Doherty]

Validation notes for this carbon/performance slice:

• git diff --check passes
• source check confirms src/pages/contribute-to-opensource.astro no longer references unpkg.com/chota
• GITHUB_TOKEN=$(gh auth token) pnpm build passes
• generated dist/contribute-to-opensource/index.html contains no unpkg or chota.min.css reference
• generated guide HTML still contains the article title and local container styling