Add Decodex App launch command

[yvette-carlisle]

Summary

• add decodex app to open the native Decodex App from the CLI
• support --bundle APP_BUNDLE and --new for staged app preview
• document that DECODEX_APP_SERVER_URL remains an explicit preview override

Validation

• cargo make check
• cargo run -p decodex --bin decodex -- app --help
• env -u DECODEX_APP_SERVER_URL cargo run -p decodex --bin decodex -- app --bundle /Applications/Decodex.app
• semantic drift audit passed manually from helper output