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

slhx-test/tests/examples_contract.rs

@@ -119,6 +119,8 @@ fn canonical_example_docs_do_not_teach_low_level_plumbing() {
         "SafeHtml::trusted",
         ".render(&",
         "addEventListener(",
+        "querySelector",
+        "querySelectorAll",
     ];
     let mut failures = Vec::new();
     scan_examples(&examples, &mut |path, text| {