The nightly build is now a distinct app called "cmux NIGHTLY" with
bundle ID com.cmuxterm.app.nightly, allowing side-by-side installation
with the stable release. The nightly appcast URL is baked into the
app's Info.plist by CI, so no in-app channel switching is needed.
- Nightly workflow: rename app to "cmux NIGHTLY", set bundle ID to
com.cmuxterm.app.nightly, hardcode nightly Sparkle feed URL, publish
DMG as cmux-nightly-macos.dmg
- Remove "Receive Nightly Builds" toggle from settings
- Remove UpdateChannelSettings enum and simplify feed URL resolution
to just use SUFeedURL from Info.plist
- Remove UpdateChannelSettingsTests (no longer applicable)
