feat(cli): list globally installed skills

[codeaholicguy]

Summary

• add ai-devkit skill list --global to scan known global environment skill paths
• add optional --env <environment...> filtering while preserving project-local list behavior
• document global listing and cover deterministic output, provenance, shared paths, validation, and command behavior

Validation

• npm test --workspace packages/cli -- --run src/__tests__/commands/skill.test.ts src/__tests__/lib/SkillManager.test.ts (73/73 passed)
• npm run lint --workspace packages/cli (0 errors; 5 existing warnings)
• npm run build --workspace packages/cli (189 files compiled)
• commit hooks: monorepo lint and tests passed across all 6 projects

Risks

• Low: listing is read-only and scans only configured paths under the user home directory.
• Feature-scoped AI lifecycle lint is unavailable because this feature has no dedicated docs/ai lifecycle documents; base structure lint passes.