chore: add the empty changeset this docs-only PR still needs

Check Changeset failed: the gate wants a changeset on every PR, and its message
says how — "an empty changeset is fine for changes that release nothing".

I had asserted in the PR description that a docs-only change needs no changeset.
That was wrong about this repo's convention: 72 empty changesets already exist,
and they are how documentation work records itself without bumping a package.
Added one in that form (empty frontmatter, description only).

The alternative the message offers — a `skip-changeset` label — would suppress
the record instead of writing it, which is the wrong trade for a change that has
something worth saying in the release notes.

Refs #4001, #4177

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0147tNF4Snk7Ry1KGt4a5PY4

Author: claude

.changeset/document-weak-pass-variants.md

@@ -0,0 +1,4 @@
+---
+---
+
+docs(ui,protocol): document the six navigation / knowledge-source variants that the #4177 variant/doc gate was only ever name-checking. `apps.mdx` enumerated nine navigation item types but gave `report`, `action` and `component` a single shared sentence instead of sections; `knowledge.mdx` named `object` / `file` / `http` in a table with no example of any, leaving the per-kind keys undocumented. Each example was parsed against the real schema (`NavigationItemSchema`, `KnowledgeSourceKindSchema`) before landing. Documentation only; releases nothing.