feat: add Replicate transcription provider

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Summary

• add a Replicate-backed transcription provider selectable with --api replicate
• document Replicate environment variables and usage notes in the README
• add unit coverage for provider output parsing, token/size validation, CLI wiring, and the unsupported --correct path

Validation

• python3 -m compileall -q src tests
• .venv/bin/python -m pip install -q -e .
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -B -m unittest discover -s tests -v
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -B -m sapat.script --help
• git diff --check

Companion implementation for the Daytona content bounty around extending Sapat with additional transcription APIs.

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Companion Daytona content PR for the bounty guide: daytonaio/content#231. The content PR cites this implementation so readers can run the Replicate provider instead of following a theoretical integration.

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I pushed a small defensive follow-up in 737273f:

• use replicate.Client(api_token=...) instead of mutating os.environ
• raise a clear error for unsupported/empty segment output instead of silently returning an empty transcript
• expand unit coverage from 7 to 10 tests, including optional Whisper model wiring, unsupported extension validation, exact CLI argument forwarding, and unsupported output shape handling

Validation run after the update:

• python3 -m compileall -q src tests
• .venv/bin/python -m pip install -q -e .
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -B -m unittest discover -s tests -v
• PYTHONDONTWRITEBYTECODE=1 .venv/bin/python -B -m sapat.script --help
• git diff --check

The external Continue status links are returning 404 for me, so I cannot inspect their private feedback directly. If visible feedback appears, I will address it quickly.

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Status note: the three red statuses on this PR are external Continue tasks reporting only "Agent encountered an error"; they are not repository tests and do not expose actionable failure details from the GitHub status API. I rechecked the implementation locally after the hardening commit: the provider path still validates with the repo\x27s Python test/import flow, and the follow-up kept the API-token handling and unsupported-api errors scoped to this PR.