[codex] Add portfolio concept pages

[YinkaMetrics]

Summary

Adds a static, hosting-neutral portfolio concept build for Adeyinka A. Akanbi.

• Adds a root concept chooser page.
• Adds a dark Builder/Product Engineer portfolio direction.
• Adds a light Data/Business Systems portfolio direction.
• Adds shared responsive styling, a small theme-toggle script, and placeholder CV download files.

Impact

This gives two complete portfolio approaches that can be reviewed side by side before choosing the final homepage for GitHub Pages or another static host.

Validation

• Parsed index.html, builder.html, and systems.html locally.
• Checked local asset references.
• Previewed desktop and mobile layouts in the in-app browser.
• Confirmed no horizontal overflow and no image load failures during local preview.

Notes

The CV files are placeholders and should be replaced with the final CV documents when ready.