fix(omzsh-plugin): require repo.json for repo detection, pre-clear state on chpwd

_muse_find_root previously accepted any .muse/ directory as a valid repo, causing false positives from stray or partial .muse/ directories in ancestor paths. Now requires .muse/repo.json to exist — the canonical marker of a properly initialised Muse repo.

_muse_hook_chpwd now pre-clears MUSE_REPO_ROOT and MUSE_BRANCH before calling _muse_refresh, so any silent failure in refresh leaves the prompt blank rather than showing stale state from the previous directory.

Adds muse_debug() for in-shell diagnosis when the prompt looks wrong.