CONTRIBUTING.md: add cargo-nextest as a documented prerequisite #92

Why

An agent running just test on a machine without cargo-nextest gets an unhelpful error and may waste time diagnosing a Rust compilation problem that doesn't exist.

Current state

just test runs cargo nextest run, which requires the cargo-nextest binary. CONTRIBUTING.md does not list cargo-nextest in its prerequisites. CLAUDE.md also makes no mention of it. Running just test without the tool produces: cargo: no such subcommand: nextest.

Ideal state

• CONTRIBUTING.md lists cargo-nextest in its Prerequisites section alongside just, taplo, and prek.
• The entry includes an installation command: cargo install cargo-nextest --locked.
• An agent or developer reading CONTRIBUTING.md can set up a complete working environment without discovering missing tools at runtime.

Starting points

• CONTRIBUTING.md — the file to update with the missing prerequisite
• justfile — confirms just test calls cargo nextest run

QA plan

1. Open CONTRIBUTING.md — expect cargo-nextest listed in the Prerequisites section.
2. Confirm an installation command (cargo install cargo-nextest --locked or equivalent) is included.
3. Run just test on a machine with cargo-nextest installed — expect tests to run without errors.

Done when

CONTRIBUTING.md lists cargo-nextest with an installation command in the Prerequisites section.