fix(zsh-plugin): refresh dirty state on every prompt, not just after muse commands

Previously the dirty check only ran when _MUSE_CMD_RAN=1 (i.e. after a muse command). This meant touch, vim, cp etc. never updated the branch colour. Now precmd always calls _muse_check_dirty when inside a repo, and does a full refresh (head + domain + dirty) after a muse command. Same model as the git plugin.