Refactor skill command UI boundary

[codeaholicguy]

Summary

• Move skill command implementation into commands/skill with an index entry point and render helper.
• Remove terminal UI, spinners, and environment prompts from services/skill.
• Return structured install/remove/rebuild results from skill services and render them in the command layer.
• Update install and skill tests to assert service reports instead of service-side console output.

Validation

• npx tsc -p packages/cli/tsconfig.json --noEmit
• npm --workspace packages/cli test -- skill
• npm --workspace packages/cli test -- install.service
• npm run build
• npm --workspace packages/cli test
• pre-commit hook: repo lint + repo tests passed

Risk

• Low: CLI output is now rendered from service result objects, so command behavior should remain equivalent while service tests no longer depend on terminal output.