chore(devshell): Create a dev env to facilitate contributions.

[vic]

Create a development environment.

[Copilot]

Pull Request Overview

This PR introduces a Nix-based development shell and updates the project README to explain how to use it.

• Adds devshell.nix to configure default devshell commands (regen, fmt, check).
• Updates README.md with instructions for entering the dev environment and a list of available commands.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dev/modules/devshell.nix	Define a flake-based devshell with three commands
README.md	Describe how to use nix develop and list commands

Comments suppressed due to low confidence (2)

README.md:267

• [nitpick] The placeholder [[general commands]] appears inside a shell code block but isn’t valid shell syntax. Consider removing it or replacing it with explanatory text outside the code block.

[[general commands]]

README.md:271

• The menu command is listed here, but it isn’t defined in devshell.nix. Either add a menu entry to the devshell commands or remove this listing.

  menu  - prints this menu