devcontainer: derive LLVM apt repo codename from base image

[pbrady]

What

The devcontainer's python:3.11-slim base image silently advanced from Debian 12 (bookworm) to Debian 13 (trixie). The Dockerfile hardcoded the bookworm LLVM apt repo:

http://apt.llvm.org/bookworm/ llvm-toolchain-bookworm-${CLANG_VERSION} main

on what is now a trixie base. This derives the codename from /etc/os-release instead:

http://apt.llvm.org/${VERSION_CODENAME}/ llvm-toolchain-${VERSION_CODENAME}-${CLANG_VERSION} main

Why it matters

The mismatch was latent, not fatal: trixie ships clang-19 in its own archive, so apt installed the whole toolchain from deb.debian.org and never actually used the wrong-codename apt.llvm.org repo. But it's dead weight that would break the moment CLANG_VERSION is bumped past what Debian packages — the fallback repo would point at the wrong Debian release. Deriving the codename fixes today's mismatch and future-proofs the next base-image bump.

Validation

• Built the base-system stage on the trixie base: apt.llvm.org/trixie now resolves (Get: https://apt.llvm.org/trixie llvm-toolchain-trixie-19 InRelease), and clang-19 / clangd-19 / clang-format-19 / lld-19 / libc++-19-dev install cleanly from deb.debian.org trixie/main.
• Rebuilt the full local devcontainer image (gcc flavor) from scratch on trixie/gcc-14: kokkos@5.1.1 present in the Spack view; t-csr and t-dense compile and pass.

No functional change to the installed toolchain (clang-19 comes from Debian either way) — this only corrects the fallback repo codename.