Migrate from npx skills to Dart skills package for skills management

Sub-issue of #201. Migrate skills management away from npx skills (.npmrc, skills-lock.json) to use the native Dart skills package.

Start with parent issue #201 and inspect the repository’s .npmrc and skills-lock.json references, then identify how the native Dart skills package is expected to replace npx skills. Done means skills management no longer depends on npx skills and the existing migration artifacts are addressed.

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