Tracker: Publish dart_skills_lint as a standalone package

[reidbaker-agent]

Tracking issue for publishing dart_skills_lint as a standalone open-source Google-quality package.

Tasks / Sub-issues

• Migrate LICENSE and copyright headers to Apache 2.0 (Google standard open-source license and header).
• Migrate skills management from npx skills (.npmrc, skills-lock.json) to the Dart skills package.
• Complete publishing review and release to pub.dev / GitHub releases.