ci(coverage): document that birpc guard covers coverage run only

Adds a comment above the assertion step so a future developer diagnosing
a birpc-related failure in `npm test` knows where to find the diagnostic.

Addresses Sara Holt + Tobias Wendt round-4 observation on PR #536.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -51,6 +51,8 @@ jobs:
         env:
           TZ: Europe/Berlin
 
+      # Diagnostic guard: covers the coverage run only. If `npm test` (above)
+      # exits 1 with a birpc error, the named pattern appears here — not there.
       - name: Assert no birpc teardown race in coverage run
         shell: bash
         if: always()