#30405 Revert "Cloud: every signed NEXT build (DEV and nightly) defaults to NEXT Cloud via the app-server link"

Summary

Reverts #28899 (dc8a4a49cc5), opened by gh-merge-green --revert.

Changelog

Changed: reverts #28899 (- Changed: signed cmux NEXT DEV and NEXT nightly use NEXT Cloud (the development Worker and app-server link) by default. Agent session: agent session 🤖 Generated with Claude Code)

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Summary by cubic

Reverts #28899, restoring the previous behavior where signed NEXT DEV and nightly builds use production Cloud instead of defaulting to NEXT Cloud.

• Signed NEXT builds return to production endpoints, production auth, and their standard Keychain and UserDefaults storage.
• The app-server link and staging sign-in are now used only by Debug builds with explicit environment overrides.
• Machine actions (kill, pause, resume, resize, rename, idle pause, snapshots, restore) go back through /api/vm instead of app server ops.
• Removed the signed-DEV configuration and auth-persistence tests added with the original change.

Takes back #28899: CmuxNextApp +94 lines turned the feat tip arch ratchet red (6be272858f7); re-land with the code moved to a package.