docs: add registry discoverability note — and category hints for first-time authors

[jdjioe5-cpu]

Context

I tried to discover existing skills before proposing a new one and the search workflow was awkward. The runx registry search subcommand isn't obviously documented at https://runx.ai/x, and the catalog page groups by category but doesn't expose category metadata in a way an agent can use to pre-filter.

Reproduction

1. Visit https://runx.ai/x
2. Click into "code" category
3. Note: there's no /x?category=code filter URL
4. runx registry search code-review returns results but the docs page https://runx.ai/spec doesn't index the search subcommand under "Registry Protocol"

Proposal

A small docs page or section that:

• Lists every runx registry <verb> subcommand with one-line purpose
• Documents the catalog URL filter convention (/x?category=<slug> if supported, otherwise a fallback)
• Calls out the relationship between the public catalog and runx registry read <owner>/<skill>@<version> (already documented, but not linked from /x)

Why this matters

The skill catalog at /x is the public adoption page. First-time agents browsing the catalog cannot reproduce the search workflow without opening a terminal. Bridging the catalog-to-CLI gap reduces the friction for new contributors.

Scope

• Docs only, no code change
• Out of scope: registry API changes, new CLI flags, marketplace pricing

Refs https://runx.ai/x (current catalog), https://runx.ai/spec (spec page)