Digital engineering (DE) and model-based systems engineering (MBSE) improve traceability for requirements, architecture, and verification, but concept decisions—the governance events that turn evolving evidence into binding commitments—are poorly captured. Rationale, assumptions, alternatives, model baselines, and approval conditions are scattered across slides and minutes, limiting auditability, reproducibility, and automation. We propose a Decision Digital Thread (DDT): a typed graph schema that makes decisions governable by linking framing and scope, structured (including set-based) alternatives, uncertainty and risk, immutable evaluation-run provenance with reviewed evidence, and commitment events with machine-actionable conditions, authorized actions, and outcome feedback. DDT serves as the decision system of record and a contract between platform modules and enterprise policy while referencing MBSE/PLM/simulation artifacts via stable identifiers and configuration context. Policy-driven readiness gates block lifecycle transitions when evaluator coverage, evidence review, or bias checks are incomplete. An electric pickup range-extension case demonstrates auditable gates, evidence lineage, and safe AI-agent authority boundaries.