feat(skills): support global skill removal

[codeaholicguy]

Summary

• add ai-devkit skill remove SKILL_NAME --global
• add --env ENVIRONMENT... filtering and reject it without --global
• preserve project-local removal behavior when --global is absent
• guard removal to direct children of configured home-relative global skill roots
• preserve registry cache and safely handle symlinks, missing entries, duplicate paths, and partial failures
• update CLI help and skills documentation

Validation

• focused command/manager tests: 74 passed
• full CLI package tests: 909 passed
• repository commit hooks: all 6 lint and all 6 test targets passed
• workspace build/type declarations: all 6 build targets passed
• post-rebase CLI build: passed
• isolated temporary-home built CLI test: symlink and copy removed; symlink target and registry cache preserved; missing skill succeeds; invalid --env usage fails
• strict TDD reverse-fix proof: selected-global-environment test failed with dispatch disabled and passed after restoration

Risks

• --global without --env intentionally targets every environment with a configured global skill root.
• partial filesystem failures can leave some selected environments removed and others intact; the command continues and exits nonzero with a summary.