Unify skill handling layer and raise the active skills cap to 20.
The Skills UI now presents a simpler checklist-style flow for selecting active
skills, with live chat activation and saved defaults using the same visible list.
Skill contents can be opened in a read-only Ace viewer via the existing markdown
modal.
Add a shared safe markdown pipeline for plugin READMEs and docs.
- vendor DOMPurify and introduce a shared safe-markdown helper
- centralize GitHub README link/image rebasing, including repo routes like `releases`
- sanitize rendered HTML before all plugin-related x-html sinks
- apply the shared renderer to Plugin Hub README, installed plugin README, and markdown modal docs
- preserve target/rel handling for external links