Add launch-at-login setting

[yvette-carlisle]

Summary

• add a macOS Login Items-backed Open at Login toggle in Settings > Permissions
• keep system permission controls first and place startup below them
• document the Settings contract and cover login item state mapping in the native host probe

Validation

• cargo make fmt-swift
• cargo make lint-swift
• cargo make test-macos-native-host
• cargo make test-macos-native-host-stage
• git diff --check
• semantic drift audit
• clean release rebuild and manual Settings smoke