webclient: read the skills catalog as documents, split it into two tabs ...

Settings had one long panel mixing skills and protocols as raw markdown,
which was hard to read and wasted the width. Skills and Protocols are now
two tabs over one panel keyed by kind.

Selecting an entry shows the parsed document — triggers, preconditions,
steps, post-checks, escape hatches, body — and an Edit button switches to
the source, so the catalog is read the way the agent loads it. The
single-entry payload carries the parsed document; the list still does not,
so it stays cheap. SkillDocument is shared with the chat proposal card,
which now only draws its own frame and header.