ci: pin realtime voice to gpt-realtime-2

[dimavrem22]

Executive Summary

This PR pins realtime voice CI to gpt-realtime-2 for every voice scenario.

• Keeps the native host model for text turns.
• Makes the voice-model invariant explicit before scenario selection.

Description

The voice workflow now exports INKBOX_REALTIME_MODEL=gpt-realtime-2 for the entire scenario matrix instead of only the realtime branch. Hosted and transcript-based scenarios continue using their existing speech stacks.

Reason

The scheduled voice suite should carry one explicit realtime model pin without changing the host-native text execution that this integration is meant to validate.

Decisions

• Text model scope: Kept text turns on the native host because substituting a different host protocol would stop validating this integration.

Testing

• Workflow configuration: Parsed every workflow YAML file successfully.
• Voice model configuration: Verified the live voice workflow contains the gpt-realtime-2 pin.
• Patch hygiene: git diff --check passed.