Add formatter targets and pre-commit hooks

[zah]

Summary

• add pre-commit-hooks flake and run just lint via git hooks
• install rustfmt in setup
• support formatting and linting via Justfile
• document development workflow
• ignore generated pre-commit config
• format Rust sources
• break format and lint tasks into per-language commands

Testing

• just test
• just lint
• just format

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https://chatgpt.com/codex/tasks/task_e_68617123183c83299933fae96564bc4c

[github-actions]

Benchmark Report

Benchmark	Ruby (no tracing)	JSON	CAPNP	JSON (PureRuby)
heavy_work	68ms	✓ 624ms 78110688B	1000ms 25149319B	✓ 35144ms 211729021B