Add Together AI transcription provider

[katlark]

Summary

• add a Together AI transcription provider using the multipart audio transcription endpoint
• expose it through sapat --api together
• document the required Together AI environment variables in the README
• add focused mocked unit coverage for request construction, transcript extraction, missing config, and CLI routing

Validation

• PYTHONPATH=src .venv/bin/python -m unittest discover -s tests -v
• PYTHONPATH=src .venv/bin/python -m py_compile src/sapat/script.py src/sapat/transcription/together.py tests/test_together_transcription.py tests/test_script.py
• PYTHONPATH=src .venv/bin/python -m sapat.script --help
• git diff --check

Companion implementation for Daytona content bounty daytonaio/content#13.

No API keys, recordings, payment details, private account data, or local sensitive paths are included.