docs(cli): update netclaw skill classifications for server feeds (cleanup)

[Aaronontheweb]

Part of #95 · Group 4 · UPDATE cli/skill.md · self-serve · low priority

Scope: add the server-feeds tier to the 'Skill classifications' table (currently only system/native/external); clarify that netclaw skill source manages external directories only (remote feeds are config/TUI-managed, no CLI add command); fix the user-invocable vs invocable field-name inconsistency with skills/overview.md.

Deps: none.