docs: add CLI coverage badge to README

Shows coverage percentage from GitLab CI pipeline.

Author: claude

README.md

@@ -3,6 +3,7 @@
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![GitLab](https://img.shields.io/badge/GitLab-postgres--ai%2Fpostgres__ai-orange?logo=gitlab)](https://gitlab.com/postgres-ai/postgres_ai)
 [![PostgreSQL](https://img.shields.io/badge/PostgreSQL-14%2B-blue?logo=postgresql)](https://www.postgresql.org/)
+[![CLI Coverage](https://gitlab.com/postgres-ai/postgres_ai/badges/main/coverage.svg?job=cli:node:tests)](https://gitlab.com/postgres-ai/postgres_ai/-/jobs)
 
 **Expert-level Postgres monitoring tool designed for humans and AI systems**