Add Dev Container for Codespaces + CI configuration

[haydnv]

Summary

• Add .devcontainer/devcontainer.json for Codespaces/Dev Containers with Rust toolchain, rustfmt/clippy, GitHub CLI, and OpenCL headers (compile-time) preinstalled.
• CI (already present) continues to build/check/test on push/PR. No OpenCL runtime tests added.

Notes

• This enables working on issues fully in the cloud without running code locally.
• No behavior changes to the library; no new tests required.

Security/Config

• No secrets added; CI does not require tokens.

Let me know if you want an additional workflow for OpenCL compile-checks only or issue automation (e.g., /close).