Per-workspace `plugins:` now UNIONS with `defaults.plugins` instead of
replacing. A leading `!` or `-` on a per-workspace entry opts a default
out. Backward-compatible: re-listing defaults still dedupes to the same
list.
Refactored the inline REPLACE logic into a pure helper `mergePlugins`
in org.go so it's unit-testable. Five TestPlugins_* cases added.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
d9603a77ce