docs: re-add goreportcard badge

it works once the project has been ingested - the earlier failure was
a cold-start race (badge cached as a 307 before generation finished).
current grade: A+

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brtkwr

README.md

@@ -2,6 +2,7 @@
 
 [![CI](https://github.com/agentic-utils/zeltapp-cli/actions/workflows/ci.yaml/badge.svg)](https://github.com/agentic-utils/zeltapp-cli/actions/workflows/ci.yaml)
 [![Go Reference](https://pkg.go.dev/badge/github.com/agentic-utils/zeltapp-cli.svg)](https://pkg.go.dev/github.com/agentic-utils/zeltapp-cli)
+[![Go Report Card](https://goreportcard.com/badge/github.com/agentic-utils/zeltapp-cli)](https://goreportcard.com/report/github.com/agentic-utils/zeltapp-cli)
 [![Release](https://img.shields.io/github/v/release/agentic-utils/zeltapp-cli?sort=semver)](https://github.com/agentic-utils/zeltapp-cli/releases/latest)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](LICENSE)