fix: wire walk_workdir into StatCache — was hashing every file on every call (#157)

walk_workdir was calling hash_file() unconditionally, completely bypassing the StatCache that was written alongside it. Now it loads the cache via load_cache(), uses get_cached() in the inner loop, prunes stale entries, and saves atomically — making every subsequent muse status / commit / diff call skip re-hashing unmodified files.

Fixes two pre-existing integration test failures in TestWalkWorkdirCacheIntegration.

Co-authored-by: Gabriel Cardona <gabriel@tellurstori.com>