fix(cache-health): fallback to most recent JSONL when session_id not provided

fix: add PATH export and pre-init vars for set -u safety

refactor: organize plugins into per-plugin subdirectories

feat: add cache-health and budget-clock plugins

feat: full customization - colors, icons, and theme support

feat: declare color slots in plugin TOML sidecars

feat: use theme colors from soffit config

feat: community plugins for soffit statusline manager