Expand contributing guide

[Flerpharos]

Summary

• expand the root CONTRIBUTING.md from the existing Sphinx contributor guide
• document unit, integration, and full-suite test commands
• add non-mutating Ruff checks and summarize the GitHub Actions quality gates
• complete the project's typing conventions and Python 3.10 compatibility guidance

Why

The repository already had a concise contributor file, but the fuller development workflow lived primarily in the documentation site. Bringing those established instructions into the root guide makes GitHub's contributor entry point more complete and keeps the commands consistent with the existing docs and configuration.

Impact

Documentation only. Contributors get a more complete local setup and verification reference; runtime behavior is unchanged.

Validation

• uv run pre-commit run --files CONTRIBUTING.md
• git diff --check
• verified documented hook IDs, workflow names, and example test path against the repository