Improve review prompting and runtime customization

[divyangchauhan]

Summary

• add a hardened review contract that limits findings to issues introduced, exposed, or materially worsened by the patch
• embed focused default review guidance with evidence, severity, uncertainty, chunking, and noise controls
• support one-run overrides through --prompt and --prompt-file, while preserving invariant safety and structured-output requirements
• add prompt behavior and CLI validation tests
• document public AI code-review approaches, design decisions, and an evaluation plan

Testing

• go test ./...
• go vet ./...
• go build ./cmd/diffvouch
• git diff --check feat/diffvouch-go-cli...HEAD

Dependency

This is a stacked PR targeting feat/diffvouch-go-cli, which is currently proposed to main in #4.