gate: zsh runs a brace group with no space after the open brace, which the subagent guard does not see as a command start #318

Found by the #306 differential fuzz (3 of its 46 remaining false-allows; pre-existing).

zsh runs a brace group written with no space after the open brace (git stash directly after it, then a semicolon and a close brace). bash, dash and ksh do not treat that as a group, so the guard's command-start anchor does not either. The Bash tool on this machine runs zsh, so this is a live subagent-guard bypass for plain stash, reset and clean today. Fix direction: accept an open brace directly followed by a command word as a command start (zsh reading; any deny wins), and fuzz it against zsh.