[codex] Set Rust-first engineering default

[donbeave]

Summary

Adds a Rust-first implementation default to the shared engineering rules so new project-owned automation, CLIs, parsers, state machines, and durable helpers default to Rust unless another ecosystem is the natural fit.

What ships

• Documents Rust as the default language for long-lived project-owned implementation work.
• Keeps explicit exceptions for docs-site TypeScript, shell entrypoints, and tiny bootstrap glue.

What this addresses

• Makes the Rust preference durable for future Jackin work instead of leaving it as chat-only guidance.

Verify locally

git clone https://github.com/jackin-project/jackin "$HOME/Projects/jackin-project/test/pr-rust-first/jackin"
cd "$HOME/Projects/jackin-project/test/pr-rust-first/jackin"
git fetch origin codex/rust-first-guidance
git checkout codex/rust-first-guidance

Review ENGINEERING.md and confirm the Rust-first rule is clear and scoped.

Migration notes

None.