refactor: consolidate MCP server configuration modules

This commit restructures the project to provide a unified approach for configuring MCP
servers across different Nix module systems.

Previously, this flake provided separate implementations for devenv and
home-manager. However, both projects now have native support for Claude configuration:

- home-manager: `programs.claude-code`
- devenv: `claude.code`

- Renamed from `claude-code.nix` to `mcps.nix` to reflect the new focus on providing
reusable MCP server configurations

Added two new module implementations that integrate with existing Claude modules:

1. home-manager module (nix/modules/home-manager/claude/)
   - Adds `programs.claude-code.mcps` key
   - Provides access to preset MCP server configurations
   - Integrates with home-manager's native Claude support

2. devenv module (nix/modules/devenv/claude/)
   - Adds `claude.code.mcps` key
   - Provides access to preset MCP server configurations
   - Integrates with devenv's native Claude support

- Migrated from bash-based integration tests to nixtest
- Added separate test suites for:
  - home-manager stable (25.05)
  - home-manager unstable
  - devenv stable

- Tests verify MCP server configuration, environment variables, and command setup

- Old test fixtures and bash-based integration test script
- Legacy module implementations that conflicted with upstream changes

This refactoring positions the project as a curated library of MCP server presets that works
alongside the official Claude modules in both ecosystems.

refactor: use nixDir to build overlays

Remove easyOverlay in favor of nixDir overlay management.