Separate cmux NIGHTLY as standalone app with its own bundle ID (#164)

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- Remove UpdateChannelSettingsTests (no longer applicable)

Sources/Update/UpdateController.swift

@@ -31,7 +31,6 @@ class UpdateController {
             "SUEnableAutomaticChecks": false,
             "SUSendProfileInfo": false,
             "SUAutomaticallyUpdate": false,
-            UpdateChannelSettings.includeNightlyBuildsKey: UpdateChannelSettings.defaultIncludeNightlyBuilds,
         ])
 
         let hostBundle = Bundle.main