test: verify Gentle AI review pipeline end-to-end

[masch]

Summary

Add a Shields.io badge to README.md to exercise the Gentle AI review pipeline end-to-end through the full SDD flow.

Change

• Insert [![Gentle AI Review](https://img.shields.io/badge/Gentle_AI_Review-Reviewed-brightgreen)](https://github.com/features/actions) as first line of README.md
• +2 lines (badge + blank line), 0 deletions

Why

This is an infrastructure/test change — no product value. Sole purpose is to verify the Gentle AI review lifecycle (start → finalize → validate → commit) works correctly on this repository.

SDD Phases Executed

• proposal → spec → design → tasks → apply → verify → archive

Review Lifecycle

• gentle-ai review start --projection staged → lineage: review-27c66589f952f9d5
• gentle-ai review finalize → approved
• gentle-ai review validate --gate pre-commit → allow
• Commit: 48f6366 on test/gentle-review-pipeline

Test Plan

• Badge renders at top of README.md
• Badge links to github.com/features/actions
• # Sonora heading preserved at line 3
• git diff shows exactly +2 lines, 0 deletions