Install observational Merge Steward

[mrjf]

Summary

• add a schema-validated merge-readiness policy for the four existing CI checks
• add a deterministic, read-only reconciler with table-driven planner tests
• add an exception-only Agentic Workflow with trusted-head preflight and staged safe outputs
• keep native auto-merge inactive while the steward runs in observation mode
• leave existing Evergreen automation unchanged and outside Steward dispatch

Rollout

The steward starts in observation mode. It does not dispatch workflows, mutate labels, update branches, post comments, or merge pull requests.