fix(zsh-plugin): re-entry guard on chpwd to stop recursion, revert env -C

env -C broke muse discovery (rc=127) because it spawns a plain process that does not inherit ZSH's venv/PATH activation. Revert to the ZSH subshell cd.

The real recursion fix: add _MUSE_REFRESHING guard at the top of _muse_hook_chpwd. ZSH subshells inherit parent variables, so when the cd inside _muse_check_dirty fires chpwd inside the subshell, the guard is already 1 and the nested hook returns immediately — no recursion.