[automated] Build a CI failure issue shepherd #19789

[automated] Track the implementation of an artifact-driven shepherd for CI failure issues under #19759.

• Define the architecture and issue-lifecycle model.
• Implement artifact-driven collection and read-only proposals.
• Add lifecycle replay state and duplicate-closure proposals.
• Complete implementation and independent evaluation.
• Open and land the implementation PR.
• Roll out in read-only mode before enabling approved mutations.