Switch to nixica 0.1.0

Summary

• Migrate from bare flake to nixica 0.1.0 infrastructure
• Use nixica.lib.mkFlake with import-tree auto-discovery
• Move flakeModule into nix/ directory with import-tree pattern
• Replace test flake's flake-parts/get-flake inputs with nixica
• Add systems configuration

Closes #1 (closed)

Test plan

• Verify nix flake show outputs flakeModule and flakeModules
• Verify nix fmt passes
• Verify test flake evaluates correctly

@locallycompact