Model structured code-quality findings in diff review

[LarryHu0217]

Summary

• define a provider-neutral QualityFinding contract, fixture provider, and deterministic stale-location reconciliation
• render current findings inline and all open findings in the existing review sidebar with textual severity/category metadata
• support dismiss, single-send, and multi-select structured feedback to the selected task agent

Validation

• npm run check
• npm run lint:dead
• npm run lint:arch
• npm test (1636 passed, 23 skipped)
• npm run build:frontend

Closes #239