Users working across multiple worktrees or repos no longer need to
re-enter personal settings (user_name, communication_language,
user_skill_level) in every project. A global user layer at
~/.bmad/config/config.user.toml is merged as the lowest-priority
fallback in both resolve_config.py (5-layer) and
resolve_customization.py (4-layer). Project-level overrides always
win. Missing global dir is fully backward-compatible.
Closes#2338
Jerome Revillard