gate: subagent guard does not see a wrapper written as a path (/usr/bin/env git stash) #320

Found by the #318 differential fuzz (pre-existing; develop allows it too).

The subagent guard's wrapper list (_WRAPPER_WORDS) matches only the bare word, so a wrapper written as a path is not a wrapper: /usr/bin/env git stash runs git stash in every shell tested (sh, bash 3.2, bash 5, dash, zsh, ksh) and the guard allows it. The anchor already takes /path/git, but not /path/env before it. 40 of the 43 branch false-allows in the #318 fuzz (4000 cases) are this shape, e.g. ( { /usr/bin/env git -C . clean -fd; } ).

Fix direction: let a wrapper word carry an optional path prefix ((?:\S*/)?env, same as the git word), in the walk and in the lead-chain lookahead, then fuzz per docs/reference/repo-gotchas.md. Check irrecoverable.py's PREFIX_WRAPPERS matching for the same gap (it may already compare a basename).