#23092: fix: make sponsor logos visible in light mode

## Summary - Sponsor SVGs (OpenAI, Blacksmith) use `fill="white"`, making them invisible on light backgrounds - Added dark-fill (`#171717`) variants of both SVGs for light mode - Updated README to use `<picture>` with `prefers-color-scheme` media queries so logos swap automatically between light and dark themes ## Test plan - [ ] View README on GitHub in dark mode — logos should appear white as before - [ ] View README on GitHub in light mode — logos should now appear dark and be visible 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds dark-colored variants of sponsor logos (`openai-light.svg`, `blacksmith-light.svg`) and updates README to use `<picture>` elements with `prefers-color-scheme` media queries. The original white logos now display in dark mode while new `#171717` (dark gray) variants display in light mode, making logos visible on both background types. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risks - Simple, focused change that only affects README presentation. The implementation uses standard HTML `<picture>` elements with proper fallbacks, and the new SVG files are exact color variants of existing assets. No logic changes, no dependencies affected, no breaking changes possible. - No files require special attention _{Last reviewed commit: 7f655c2} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->