Use app helper for Codex account switching

[yvette-carlisle]

Summary

• replace the Swift app CLI wrapper with a bundled decodex-app-helper JSON bridge
• extract reusable Rust account service APIs for list/login/select/logout/use-in-Codex
• stage and sign the helper inside the Decodex App bundle

Verification

• cargo make check
• cargo make test-decodex-app-stage
• helper JSON smoke with temporary HOME
• git diff --check
• semantic drift audit: pass