refactor(repo): simplify repository and stabilize linux cuda builds

[LessUp]

Summary

• remove AI governance and non-core benchmark/pagerank surfaces
• simplify docs/workflows and keep a single root CHANGELOG
• add official Linux CUDA presets and fail-fast Conda host compiler guidance

Validation

• cmake --preset cuda-linux && cmake --build --preset cuda-linux && ctest --preset cuda-linux
• cmake -S . -B build-no-cuda -DSPMV_REQUIRE_CUDA=OFF && cmake --build build-no-cuda && ctest --test-dir build-no-cuda --output-on-failure
• cd docs && npm run verify:site && npm run build