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Sync header + §12 changelog summarising the 18-task epic «Observer instrument expansion v2» implementation. Each subsection (§12.1-§12.9) references the brain-retro 2026-05-20 #N item and the worktree commit chain. Closes Task 20 of docs/superpowers/plans/2026-05-20-observer-instrument-expansion.md. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>