docs: add codebase de-duplication implementation plan
18 tasks across 4 phases:
- Phase 1: OrderType, ConfigError, TLS types (zero risk)
- Phase 2: ErrorCategory, ModelType, re-export cleanup (low risk)
- Phase 3: Dead code audit, config warnings, Adam move (low risk)
- Phase 4: CircuitBreaker trait + inline CB replacement (medium risk)
Scoped out: ErrorSeverity (semantically different variants),
RetryStrategy (complex merge), trading_engine/risk CBs (domain-specific)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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