Remove nightly build, add OTA fallback builds to QA Team with changelog

The nightly native build is redundant now that changes ship over the air:
the only time testers need a new native build is when the OTA workflow
detects a native fingerprint or version change and falls back to building.

Move the useful parts of the nightly into that fallback path instead:

- Assign the fallback iOS build to the "QA Team" TestFlight group
- Generate a changelog of commits since the last native testflight build,
  tracked via a "testflight-native-build-commit" artifact that only
  advances after both native builds succeed
- Pass the changelog as the TestFlight "What to Test" notes (new
  `changelog` input on build-submit-ios.yml for workflow_call)
- Post the iOS and Android builds with the changelog to Slack

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

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

@@ -33,6 +33,11 @@ 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)