[codex] Add system runbook endpoint

[LioRael]

Summary

• Add the GET /admin/data/service-system/runbooks admin data endpoint.
• Add DTOs and OpenAPI contract output for system runbook summaries.
• Document the generated-first system runbook plane and its control-plane boundary.

Test Plan

• cargo test -p lenso-platform-admin-data
• just generate-contracts
• just generated-check
• just arch-check

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Latest commit: 1943647

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