cmux test bisect

PR CI runs selected tests, so a package suite can stay red on main for weeks with nobody noticing. This skill finds the commit behind each failure on CI, never on the Mac (no full cmux builds on laptops).

App-host (cmuxTests) failures on main have their own automatic bisect (#14510). Use this skill for the SwiftPM package suites under Packages/iOS and Packages/Shared that test-ios.yml can run (CMUXMobileCore, CmuxSyncStore, CmuxMobilePairedMac, CmuxMobileChanges, CmuxMobileShell, CmuxMobileShellModel).

Before you start