Parallel iOS test agents: give each xcodebuild test run its own simulator

When several coding agents build and test the same iOS app at once (separate git worktrees, one per stage), do not point two xcodebuild test runs at the same simulator UDID. Two test runs on one simulator fight over install, launch and boot state, and one run fails or hangs.

What worked: - Make one simulator per agent: xcrun simctl create "<name>" "iPhone 16" com.apple.CoreSimulator.SimRuntime.iOS-18-6 prints the new UDID. - Put that UDID in each agent's brief (-destination 'id=<udid>'), and list the UDIDs the agent must not touch. - Each worktree gets its own DerivedData by default (keyed by project path), so builds do not collide. Only the simulator needs to be split. - A fresh reviewer agent that re-runs tests also needs its own simulator. If you spawn it while a builder agent uses the same simulator, send it a new UDID at once. - Never run xcrun simctl shutdown all from an agent: it kills the other agents' runs.

Disk cost is small (a new simulator is a few hundred MB until apps are installed).