chore: migrate dev tasks to just

[WH-2099]

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Related Issue

Closes #166

Summary

• Replace the Makefile with a justfile that preserves the existing development recipes and adds a default recipe list.
• Update prek hooks to use just tc, validate prek.toml, and keep config validation in just check.
• Update PR and release workflows to install just through extractions/setup-just and run just check / just build.
• Refresh README and CONTRIBUTING references, requirements, and tool links for the new workflow.
• Validation: uv run prek validate-config prek.toml, uv run prek list -c prek.toml, just --fmt --check, just check.

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation