What a policy gate catches in AI-generated code, and what slips through

I maintain an open-source GitHub Action called vorsken. It does one thing: scan the diff on a pull request with Semgrep, apply a fixed policy, and return BLOCK, FLAG, or PASS. No dashboard, no model that drifts over time. Rules at ERROR/HIGH/CRITICAL severity block the merge, WARNING/MEDIUM flag it, the rest pass. Same diff, same verdict. The usual pitch for a tool like this is that it catches the SQL injection your AI assistant wrote. I wanted to see what it actually catches against real assis...