docs: RELEASE.md §5 step 2 reads the latest scheduled nightly on the pinned SHA, dispatching only if there is none #2175

Follow-up from the #2171 seat review (aprender-clerk). RELEASE.md §5 step 2 tells the release operator to dispatch ci.yml and wait for the run. Since #2167 the full ci workflow runs nightly on main, so on a frozen Friday the pinned SHA usually already has a scheduled run. Read that run first; dispatch (and wait) only when none exists on <PINNED_SHA>.

Docs only, plus any test that pins RELEASE.md facts. Refs #2167 #2170. Epic #2124.