docs(ci): document protected status contexts

.github/workflows/ci.yml

@@ -1,3 +1,7 @@
+# Keep workflow name stable.
+# Branch protection and external CI references rely on:
+# - CI / verify (push)
+# - CI / verify (pull_request)
 name: CI
 
 on:
@@ -12,6 +16,7 @@ permissions:
 
 jobs:
   verify:
+    # Keep job id stable. Renaming it changes the status context.
     runs-on: ubuntu-latest
     timeout-minutes: 20