feat(reviewer): gate on complete current-head evidence

[seonghobae]

Summary

• initialize CodeGraph in the checked-out target and fail closed when evidence is unavailable
• collect every current-head failed check log, code-scanning alert, Dependabot package/CVE, review thread, review body, and conversation comment
• deterministically block approvals on failed checks or Medium+ security/dependency findings
• preserve exact evidence collection failures instead of silently treating them as empty

Verification

• uv run python -m pytest — 72 passed, 100% line/branch coverage
• uv run python -m interrogate --fail-under=100 -q noema_reviewer — 100% docstring coverage
• git diff --check