docs: record how to restore the deferred io tier

Author: CodeJuggernaut

browser_tests/tests/customNodes/README.md

@@ -79,6 +79,20 @@ tier needs no extra assets - the pairing generator derives everything from
 under `browser_tests/assets/customNodes/` and set the tier. Reuse existing
 repo media assets - do not commit new binaries.
 
+## Deferred: the io tier (T2a)
+
+A third run tier existed that loads a workflow containing assertion nodes
+("Assert Executed" style checker nodes) and requires them to pass. It was
+removed because the assertion-nodes pack it depends on is not published yet,
+so the tier could only ever report "skipped" on every backend. When that
+pack ships, restore the tier with:
+
+```bash
+git revert e263d54f4
+```
+
+and add the assertion workflow per pack plus `"io"` to its manifest tiers.
+
 ## Gotchas
 
 - Do not run with `--trace on` against system Chrome