Add GPU MODE Cholesky sprint

[robobryce]

Adds a EuroPython sprint for the new GPU MODE Cholesky challenge, led by Bryce Adelstein Lelbach.

The proposal highlights:

• the single-file Python submission format and PyTorch, Triton, or embedded-CUDA paths
• the live B200 challenge and hosted compute, with no local GPU required
• a newcomer-friendly path from setup to a first valid leaderboard score
• an optional agentic edit-test-benchmark workflow with human direction and correctness checks

The mandatory local pre-commit check exposed existing Header.astro nullability errors. This PR includes a narrow, behavior-preserving typing fix so the hook can complete; the sprint content itself is isolated in src/content/sprints/gpu-mode-cholesky.md.

Validation:

• pnpm astro check
• pnpm exec prettier --check src/content/sprints/gpu-mode-cholesky.md
• pnpm exec astro build --mode production
• pnpm pagefind

🖼️ Preview available 🖼️ : https://ep-website--1829.org.readthedocs.build/

[nikoshell]

I'm fine with all changes that I see. Thanks for PR.

[nikoshell]

I will continue review when you change it from draft to ready for review.

[brycelelbach]

What day did you propose this for?

[robobryce]

I intended this for both sprint days: Saturday 18 and Sunday 19 July. The proposal currently follows the site-wide sprint schedule without repeating the dates in the card. I can make both days explicit in the proposal if that is useful.

[brycelelbach]

Make it non draft, looks good to go.

[brycelelbach]

@nikoshell please review

[brycelelbach]

When will this be live on the website?