* refactor: move fs-backed cache under WINDMILL_DIR Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> * feat: add WINDMILL_CACHE_PREFIX env var for per-session cache isolation Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> * feat: auto-use WEBMUX_BRANCH as cache prefix for session isolation Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>