Simplify repository structure in preparation for module system

Summary

This MR simplifies the gitlab-ci repository structure in preparation for implementing the module system architecture described in issue #2.

Changes:

• Rename modules/ directory to nix/ to match standard Nix flake conventions
• Consolidate 6 separate treefmt configuration files into a single nix/treefmt.nix
• Move flake.lib.echoGitLabCI from echo-gitlab-ci.nix to nix/lib.nix
• Move flake.flakeModule from exports.nix to nix/gitlab-ci.nix
• Rename default.nix → gitlab-ci.nix (core flake-parts module)
• Rename gitlab-ci.nix → ci.nix (self-hosting CI configuration)
• Update all flake.nix files to reference ./nix instead of ./modules
• Update test fixtures to use nix/ directory structure

This simplification will make the codebase cleaner and prepare it for the planned module system implementation.

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