Rubric discovery: suggest saving new verification patterns

[adagradschool]

Summary

After a loop completes, if the checker criteria don't match any existing rubric in rubrics/, suggest crystallizing it into a reusable rubric file.

Motivation

Rubrics should grow organically from real usage rather than being pre-designed. When a user writes an inline --checker that works well, the system should offer to save it so future tasks can reuse the same verification pattern.

Proposed UX

Loop accepted at iteration 2.
This verification pattern doesn't match any rubric in rubrics/.
Save as a new rubric? [y/N]: rubrics/ship-pr.md

Design Notes

• Compare the current rubric's gates/criteria against existing files in rubrics/
• Exact match → skip suggestion
• No match → suggest saving
• Partial match → suggest as a variant
• Only suggest after successful loops (accepted verdict)
• Sugar-created rubrics (--checker "pytest tests/") are good candidates since they started inline

See notes/d6-verifier-nlu.md "Future Work" section for additional context.