XY-851: Promote Decodex loop-runtime contract into specs and decisions

[yvette-carlisle]

Summary

• add a normative loop-runtime spec for natural-language research, promotion, internal Execution Program state, phase-scoped goals, review boundaries, unattended execution, guardrails, and harness feedback
• add the decision record for keeping graph semantics internal behind a conversational surface
• route the new contract from README, docs indexes, runtime, lane-control, review orchestration, and operator-control reference docs

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test