skills: agent-written skills and protocol proposals with chat buttons ...

A skill the agent writes now lands in the catalog on its own; a protocol
never does. `skills(op="propose")` stores the document in `skill_proposals`
and returns a card through the tool result's `ui_component` metadata — no
`render_component` call, so profiles without `navi_ui` get the card too.

The decision is the server's: the browser sends `skill_proposal` over the
WebSocket, `SkillStore.decide_proposal` writes the catalog row from the
*stored* document, names the clicking user as the author, and answers with
`skill_proposal_decided`. The agent learns the outcome as a hidden user
message, the same path a `navi_ui` form submission takes.

`create`/`update` refuse protocol documents and point at `propose` with
`replaces_id`, so the version history of a protocol always starts from a
user's yes. The chat store now renders `ui_component` from any tool whose
result carries one, not only from `mcp__navi_ui__render_component`.