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Closes brain-retro 2026-05-20 #11 — parseReasoningTag extracts opt-in <!-- reasoning: triggers="..." candidates="..." boundaries="..." --> HTML-comment from assistant text. Semicolon-separated values merged into heuristic-derived primary_rationale arrays via Set-dedupe. Conservative: tag is opt-in; heuristic still runs even when tag present (heuristic provides baseline, tag enriches). 5 new vitest tests, 309/309 GREEN. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>