docs: fix broken CI badge in README

The root README CI badge pointed at a nonexistent workflow (ci.yml), so
it rendered as an error. Point it at the repo's actual PR workflow
(pull_request.yml).

Release-trigger automation for the KMP/npm/JSR publishers is handled
separately in #1014.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jamesarich

README.md

@@ -1,6 +1,6 @@
 # Meshtastic Protobuf Definitions
 
-[![CI](https://img.shields.io/github/actions/workflow/status/meshtastic/protobufs/ci.yml?branch=master&label=actions&logo=github&color=yellow)](https://github.com/meshtastic/protobufs/actions/workflows/ci.yml)
+[![CI](https://img.shields.io/github/actions/workflow/status/meshtastic/protobufs/pull_request.yml?branch=master&label=actions&logo=github&color=yellow)](https://github.com/meshtastic/protobufs/actions/workflows/pull_request.yml)
 [![CLA assistant](https://cla-assistant.io/readme/badge/meshtastic/protobufs)](https://cla-assistant.io/meshtastic/protobufs)
 [![Fiscal Contributors](https://opencollective.com/meshtastic/tiers/badge.svg?label=Fiscal%20Contributors&color=deeppink)](https://opencollective.com/meshtastic/)
 [![Vercel](https://img.shields.io/static/v1?label=Powered%20by&message=Vercel&style=flat&logo=vercel&color=000000)](https://vercel.com?utm_source=meshtastic&utm_campaign=oss)