fix: stabilize scheduled CI retrieval gates

[Kirachon]

Summary

• Force the required retrieval lane to use the offline local-native provider so hosted CI never calls the unavailable openai_session path.
• Keep nightly benchmark regressions report-only, matching the CI contract, while allowing downstream diagnostics to run.
• Exclude generated/evidence trees and frozen corpora from source-ranking quality evaluation; add regression coverage.

Validation

• 42 focused CI/provider tests passed.
• TypeScript build passed.
• Both workflow YAML files parsed successfully.
• Fresh origin/main-shaped workspace: retrieval quality, shadow canary, and required-lane catch-rate gates all passed.

[github-actions]

Code Review Summary

• Risk: 2/5
• Classification: infra
• CI gate: PASS

Classified as infra. Risk 2/5. 4 files changed (+19/-2). 1 deterministic findings.

Findings

• MEDIUM PRE003 (infra) at .github/workflows/perf_gates.yml:1: Configuration or CI-related files changed