feat: Composable eval construction from registry graders

[spboyer]

Migrated from spboyer/waza#389

Summary

Design how users construct eval.yaml from a catalog of shared, versioned grader definitions.

Concept

Users should be able to:

1. Browse available graders (waza registry search 'factual')
2. Add a grader to their eval (waza registry add github.com/waza-evals/fact@v1.0)
3. Reference it in eval.yaml with local config overrides
4. Run evals that mix local and remote graders seamlessly

Design Questions

• What does waza registry search look like? Metadata format?
• How do remote graders interact with local graders in the same eval?
• Can you override config on a remote grader (e.g., change threshold)?
• How does waza init scaffold with registry graders?

Parent: #385