feat: add skill installer with install/uninstall and settings UI

[chriswritescode-dev]

Adds backend routes and service for skill installation, a frontend SkillInstallDialog component, shared schemas and types, and comprehensive tests for the full skill install/uninstall/list flow.

Files

• backend/src/routes/settings.ts
• backend/src/services/skills.ts
• backend/test/routes/settings-skills-install.test.ts
• backend/test/services/skills.test.ts
• frontend/src/api/settings.test.ts
• frontend/src/api/settings.ts
• frontend/src/api/types/settings.ts
• frontend/src/components/settings/SkillInstallDialog.tsx
• frontend/src/components/settings/SkillsEditor.test.tsx
• frontend/src/components/settings/SkillsEditor.tsx
• shared/src/schemas/skills.ts
• shared/src/types/index.ts