Consolidates ghost-writer/resolve-todos/tldr-this into doc-help,
hr-manager/interview into common, and the ckb-*/meeting-notes/meeting-prep/quiz
skills into cascade-knowledge-base (renaming the latter three to ckb-meeting-notes,
ckb-meeting-prep, ckb-quiz for naming consistency). Updates marketplace.json and
README.md to match the new plugin set.
