v0.5.19

Changelog

• 2d68893 Add GitHub import: airskills add github.com/owner/repo
• 43742b8 Add airskills-guide default skill
• 6cd74d4 Add missing files: createSkillWithGitHub API method + context slug tests
• 96fdab9 Add namespaced skill directories: {context}-{slug} format
• 49e4a80 Add show alias, progressive @ refs, descriptions in list
• f446411 Auto-install airskills guide on first sync
• 89a3b0d Reject invalid SKILL.md frontmatter on push
• c2ba8a8 Use URL path for skill selection: github.com/owner/repo/skill-name