fix(worker): baseline job을 full-history scan으로 처리

[pureliture]

What

• baseline queue job을 gitleaks full-history scan으로 처리하도록 scan-worker 옵션을 분기했습니다.
• baseline sentinel commit이 finding context에 덮어써지지 않도록 했습니다.
• baseline worker regression test를 추가했습니다.

Why

• personal-prod 초기 baseline은 commit_sha=baseline sentinel을 쓰는 queue job이라, 기존 단일 commit log opts(baseline^!)로 실행하면 runtime baseline이 실패합니다.

Validation

• uv run pytest tests/test_scan_worker.py -q
• uv run pytest -q
• uv run python -m governance.autopilot_gate --base origin/main

[gemini-code-assist]

Code Review

This pull request refactors the scan worker to handle baseline scan jobs differently from incremental ones by introducing helper functions to configure scan options and finding contexts. The review feedback highlights a potential backward-compatibility issue where legacy baseline jobs in the queue might decode with an incremental job type but retain a "baseline" commit SHA, which would cause runtime failures. It suggests checking both the job type and the commit SHA to identify baseline jobs, and adding a corresponding test case to verify this behavior.

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