Add idiomatic CONTRIBUTING.md for Rust server contributors

## Objective

Purify and humanize `CONTRIBUTING.md` and related contributor documentation so it reads naturally, eliminates AI boilerplate, and accurately reflects the exact current tooling, development scripts, architecture, and CI workflow of the project.

## Context

Contributor documentation should feel natural, practical, and welcoming. Boilerplate jargon, inflated claims, and stale instructions increase contributor friction. The guide needs to be a concise, direct, human-written reference tailored specifically to how this Rust server is built, tested, linted, and reviewed today.

## Deliverables

- [ ] **Human-centered rewrite:** Rewrite `CONTRIBUTING.md` using clear, pragmatic, human-written phrasing free of AI filler words, corporate jargon, and generic boilerplate.
- [ ] **Toolchain & Setup:** Provide direct, accurate setup instructions for the Rust toolchain (Rust edition 2024 / MSRV), cargo tools (`cargo-nextest`, `cargo-deny`, `cargo-audit`), and optional container services.
- [ ] **Day-to-day workflow commands:** Document the exact commands used locally that mirror GitHub CI:
- Unit & integration tests: `cargo nextest run --all-features`
- Linting & formatting: `cargo fmt --check`, `cargo clippy --all-targets --all-features -- -D warnings`
- Local CI harness: `bash scripts/local-ci.sh`
- [ ] **Architecture guidelines:** Briefly explain how to extend the server following the ports and adapters architecture without touching core domain types unless required.
- [ ] **PR & Git standards:** Detail Conventional Commits formatting, atomic PR expectations, test coverage expectations, and documentation updates.
- [ ] **Clean links:** Provide accurate links to relevant documents.

## Acceptance Criteria

- [ ] `CONTRIBUTING.md` reads naturally and is straightforward for both new and experienced Rust contributors.
- [ ] All listed commands and workflow steps match the actual scripts and CI definitions in `.github/workflows/`.
- [ ] Passes markdownlint validation cleanly.

## Verification

- [ ] `markdownlint-cli2 "**/*.md"`
- [ ] Review tone and structure against human writing guidelines
- [ ] Smoke-test listed contributor commands in a fresh checkout