LinkedIn Quietly Migrated From ProseMirror to Quill — and Broke Every Browser Automation Tool That Touched the Composer

I shipped a fix to my MCP server last week for LinkedIn's ProseMirror composer. It worked. Two days later, every LinkedIn post automation broke. This is the post-mortem of what changed, how I figured it out, and why "automate the platform" stories almost always end this way. The symptom was specific