claude-code: adds option to configure skills

[t-monaghan]

Description

claude-code can be extended with skills. This option allows for defining claude-code skills in home-manager. I've followed the existing home-manager options for claude-code to have both inline/path reference, and a "skillsDir" option.

claude-code skills docs: https://code.claude.com/docs/en/skills

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News