fix(scorer): make git-age tests month-start safe

[sorted-ai-bot]

Summary

• Fix git-age test date construction so month-start runs do not lose a whole month.
• Reuse the safe git-age anchor for temporary git commit dates in scorer tests.
• Test-only change; production scorer logic is unchanged.

Verification

• go test ./internal/scorer -run TestGitAgeBonus -count=1
• go test ./...
• go vet ./...
• go build ./...

Notes

• Public Distil repo: no private Go R&D content added.
• This is the CI unblocker needed before PR fix(discovery): exclude build-wrapper infra paths in git-mode #2 can go green.

[sorted-ai-bot]

@YumaKakuya PR #3 merge request