Add `just doctor` recipe for environment diagnostics

[bBlazewavE]

Summary

A just doctor recipe that performs deeper diagnostics than just status — checking versions, disk space, Nix store health, and common misconfigurations.

Suggested checks

• Nix version and daemon status
• Node.js version matches expected (22.x)
• Neovim health (:checkhealth output)
• Disk space for Nix store
• Shell (zsh) version
• Terminal true-color support
• Nerd Font detection
• API keys set (ANTHROPIC_API_KEY, etc.) — presence only, not values

Example output

=== Doctor ===
  ok: nix 2.28.1 (daemon running)
  ok: node v22.22.1
  ok: nvim v0.11.6
  ok: zsh 5.9
  warn: ANTHROPIC_API_KEY not set
  warn: no Nerd Font detected