Combines skill directories with proper priority ordering and provenance: workspace, then
configured, then defaults, then installed artifacts - so a directory the user controls always wins.
First occurrence of a path wins on duplicates; later occurrences (regardless of tier) are dropped.
Combines skill directories with proper priority ordering and provenance: workspace, then configured, then defaults, then installed artifacts - so a directory the user controls always wins. First occurrence of a path wins on duplicates; later occurrences (regardless of tier) are dropped.