Public preview CLI surface for prompt voice agent init

Prompt voice init is private preview in PR #9364. Before public preview, revisit and document the non-interactive CLI surface.\n\nTasks:\n- Unhide/document prompt voice non-interactive flags as appropriate.\n- Restore prompt-voice-specific help text for model selection if we keep that behavior.\n- Decide whether to expose voice selection as a CLI flag or keep customization in azure.yaml.\n- Define accepted voice values / validation / docs, or link to the service contract.\n- Run the normal CLI/spec review for the public surface.

Start with PR #9364 and the prompt voice init CLI surface, then run the normal CLI/spec review referenced in the issue. Done means the non-interactive flags and model-selection help are settled and documented, voice selection and accepted values are defined or linked to the service contract, and the public-preview behavior is validated.

Written by the indexing model from the issue text.