Replace nightly build with a nightly changelog post

Native nightly builds are redundant now that changes ship over the air,
so the nightly workflow no longer builds anything. It now only posts the
list of commits since the previous post to the QA team's Slack channel,
and posts nothing when there are no new commits. The commit marker
artifact keeps its name so the first run continues from the last nightly.

The OTA workflow's fallback native iOS build is assigned to the "QA Team"
TestFlight group so testers still get native builds when one is needed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0157o74F6wX3aTghhc7DLbzd

.github/workflows/build-submit-ios.yml

@@ -33,11 +33,6 @@ on:
         type: string
         description: TestFlight group to assign the build to after submitting ("none" to skip)
         default: none
-      changelog:
-        type: string
-        description: TestFlight "What to Test" notes (only applied when a group is selected)
-        required: false
-        default: ''
       runner:
         type: string
         description: Runner for the build job (defaults to macos-26-xlarge)