Switch sag to MiniMax TTS

[FrieSei]

What changed

• Replaced ElevenLabs CLI integration with MiniMax TTS (new client, updated flags/envs, and prompt guidance).
• Added MiniMax client package and updated voice listing and TTS request mapping.
• Updated docs (README/spec/prompting) for MiniMax usage and defaults.
• Adjusted tests and added in-memory HTTP handler utilities to avoid local socket binding in restricted environments.

Testing

• GOCACHE=/tmp/go-build GOMODCACHE=/tmp/go-mod go test ./... -mod=vendor

Notes

• Playback remains MP3-only; WAV/FLAC/PCM output requires --no-play.
• Some tests now use an in-memory round-tripper to avoid httptest.NewServer due to localhost bind restrictions.