Add Linux support (x86_64-linux, aarch64-linux)

[bBlazewavE]

Summary

The flake currently only targets macOS (aarch64-darwin, x86_64-darwin). Adding Linux systems would make Shellsmith usable on Linux workstations, WSL2, and cloud dev environments.

Changes needed

• flake.nix: add x86_64-linux and aarch64-linux to eachSystem
• bootstrap.sh: handle Linux package managers for Xcode CLI alternatives (build-essential, etc.)
• justfile: no changes expected (recipes are POSIX-compatible)
• shell/zshrc_block.zsh: verify Nix daemon path works on Linux

Acceptance criteria

• nix develop --command just setup works on Ubuntu 22.04+ and Fedora 38+
• just status shows all green on Linux

[bBlazewavE]

Taking a look at this — Linux support is a great idea but requires significant testing on multiple distros and build environments. This is on the backlog; I'm prioritizing simpler improvements first (CI, diagnostics, config overrides). If you have specific pain points on Linux, please share and I can prioritize accordingly.