fix(discovery): empty repo poll을 incremental fresh로 처리

[pureliture]

What

• refs가 없는 repo를 successful discovery poll에서 incremental fresh로 advance합니다.
• empty/no-HEAD repo regression test를 추가했습니다.
• touched discovery test/module에 ruff 정리를 적용했습니다.

Why

• personal-prod seed 중 empty repo는 baseline scan은 완료되지만 incremental commit job을 만들 수 없어 freshness evaluator가 계속 stale breach로 남겼습니다.

Validation

• uv run ruff check src/security_scanner/runtime/incremental_discovery.py tests/test_incremental_discovery.py
• uv run pytest tests/test_incremental_discovery.py -q
• uv run pytest -q
• uv run python -m governance.autopilot_gate --base origin/main

[gemini-code-assist]

Code Review

This pull request updates the incremental discovery process to advance the repository health status even when no new references are found, along with adding a corresponding test case. The reviewer recommended calling request.store.advance_repo_health directly instead of using getattr to dynamically check for the method, as it is a required part of the IncrementalScanStore protocol and dynamic checking could silently mask incomplete store implementations.

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