Write CONTRIBUTING.md and development guide

[mroczect]

Summary

Create a comprehensive CONTRIBUTING.md file and a development guide to help new contributors understand the project structure, coding conventions, and workflow.

Context

A clear contribution guide lowers the barrier for new contributors and ensures consistency.

Tasks

• Write CONTRIBUTING.md covering:
  • Project architecture (handler, core, plumbing, porcelain).
  • How to set up the development environment.
  • How to run tests, benchmarks, and fuzzers.
  • Code style guidelines (Rustfmt, Clippy).
  • Documentation conventions (link to docs/CONVENTIONS.md if exists).
  • How to submit issues and pull requests.
  • Git commit message guidelines.
  • Code of conduct reference.
• Add a link to CONTRIBUTING.md from README.md.
• Ensure the guide is accurate and up-to-date with current tooling.

Acceptance Criteria

• CONTRIBUTING.md exists and is linked in README.
• The guide is clear enough for a new contributor to follow.

Dependencies

• None, but best done after core conventions are established (Fase 0 and 5.5).