docs(kanban): remove selector example

Replace the last querySelector mention in the north-star walkthrough with selector-free prose and extend example docs contracts to forbid selector APIs.

req: examples/003

req: runtime/001

examples/kanban.md

@@ -55,7 +55,7 @@ Notes on keyed scopes:
 
 - `h-for="column in &self.columns" h-key="column.id"` — **required** for slhx-addressable nodes inside
 - `h-for="card in &column.cards" h-key="card.id"` — **required**
-- A slot inside a keyed loop is addressed as a generated keyed resource, never `querySelector(".card:nth-child(3)")`
+- A slot inside a keyed loop is addressed as a generated keyed resource, never by selector strings or positional DOM targeting
 - Without `h-key`, slhx rejects the build — no runtime selector fallback
 
 ---