fix: bump security-guard max-turns from 6 to 10

The agent was hitting the 6-turn limit when tool calls returned
unexpected results, leaving no turns for recovery.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox

.github/workflows/security-guard.lock.yml

@@ -11,7 +11,7 @@ permissions:
   issues: read
 engine:
   id: claude
-  max-turns: 6
+  max-turns: 10
 tools:
   github:
     mode: gh-proxy