ci(blacksmith): add blacksmith-testbox workflow for codex-rs

[andrei-hasna]

Summary

Adds .github/workflows/blacksmith-testbox.yml so Blacksmith Testboxes can be warmed for this repo.

Today blacksmith testbox warmup fails on codewith with "does not contain a testbox step" — no workflow here declares the useblacksmith/*-testbox actions. This adds one, modelled on the working pattern in hasnastudio/platform-alumia-legacy-3.

What it does

• workflow_dispatch with a testbox_id input (how warmup attaches to the run)
• useblacksmith/checkout → useblacksmith/begin-testbox → setup → useblacksmith/run-testbox last (holds the box open)
• blacksmith-16vcpu-ubuntu-2404 (60 GB) — codex-rs needs the RAM/disk
• Pinned Rust 1.95.0 toolchain matching rust-ci.yml, plus just + cargo-nextest
• Persists the toolchain env to ~/.bashrc / ~/.profile / /usr/local/bin — testbox login shells do not inherit job env, so without this cargo/just are not on PATH
• Prefetches the cargo registry and optionally pre-cargo checks the workspace, so an attached box is usable immediately instead of cold-compiling for 10+ minutes

Verification

• YAML parses; 1 job (light-checks-testbox), 9 steps, runs-on: blacksmith-16vcpu-ubuntu-2404
• Action SHAs copied verbatim from the known-good alumia workflow; dtolnay/rust-toolchain + taiki-e/install-action SHAs copied from this repo's rust-ci.yml
• Secrets scan on the staged diff: 0 findings
• Not yet executed end-to-end: warming a box requires the Blacksmith app on the hasna org and an API token scoped to that org (current CLI token is hasnastudio-bound). Once both exist:

  blacksmith testbox warmup .github/workflows/blacksmith-testbox.yml --job light-checks-testbox --idle-timeout 30
  blacksmith testbox run --id <id> "just clippy"

Notes

• Workflow-only change; no product code, no effect on existing CI (dispatch-only trigger).
• just check-fast / just test-fast-target referenced in some older docs do not exist in this repo's justfile — the real recipes are just clippy, just test, just fmt-check.

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