fix(studio): forbid ext:: remotes; type the per-dialect test table

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/feature-studio/src/server/git.ts

@@ -11,6 +11,7 @@ const BASE = [
   "-c", "merge.conflictstyle=merge",
   "-c", "pull.rebase=false",
   "-c", "init.defaultBranch=main",
+  "-c", "protocol.ext.allow=never", // author-supplied remote URLs must never run commands
 ];
 
 function env(who?: Identity): Record<string, string> {