Move bdc-workflow.sh entrypoint logic to Python

[amc-corey-cox]

Problem

The BDC workflow entrypoint (scripts/workflow/bdc-workflow.sh) has grown to include slug parsing, input validation, path auto-detection, and git operations — all untestable in the current bash form. We can only verify this logic by running full SB tasks.

Proposal

Replace bdc-workflow.sh with a thin bash stub that calls uv run python on a Python module within the repo. The stub handles only stderr capture and delegates everything else to testable Python code.

The Python module would cover: arg parsing, slug parsing/validation, repo clone/checkout, trans-spec path resolution, and make pipeline invocation. Each piece gets unit tests.

The uv environment is pre-built in the container (uv sync --frozen at build time), so uv run at runtime has negligible overhead.