Update stale changelog-cap comment

notes no longer feeds a TestFlight changelog (it only goes to Slack now), so
justify the 3900-char cap by Slack message size instead of the TestFlight
"What to Test" limit. Behavior unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/nightly-build.yml

@@ -62,7 +62,7 @@ jobs:
           if [ -z "$notes" ]; then
             notes="Nightly build — no new commits since the last nightly."
           fi
-          # Cap the whole changelog (TestFlight "What to Test" is limited to 4000 characters).
+          # Cap the whole changelog to keep the Slack message a reasonable size.
           # head -c caps the combined stream; cut -c would only cap each line independently.
           notes=$(printf '%s' "$notes" | head -c 3900)
           {