docs(roadmap): add feature-development agentic system + cross-link

Forward counterpart to the audit/remediation system: a DAG-scheduled pipeline
that works the planned phases (phases.md) and research questions
(research-track.md), with a port-vs-research router, a Theorist/Spec-author
(Opus) that derives discrete formulas + designs validation, a research
spike→go/no-go gate, an item-type-specific validation strategy (golden oracle /
invariant / convergence / cross-library), DAG prerequisite gating (chains A/B),
and reuse of the audit system's Integrator/CI/review machinery.  The two systems
compose into a loop (build forward, audit back).  Reciprocal cross-link added to
agentic-system-design.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

doc/reviewer/agentic-system-design.md

@@ -10,6 +10,9 @@ Sonnet did validation/error-handling, **Opus** did the numerics + the **review g
 that validated the cheaper models' work, then diagnosed a CI OOM and sequenced the
 #36→#39 rebase. The system below generalises exactly that flow.
 
+> **Companion:** the *forward* pipeline that builds the library (phases, port, research) is [`../roadmap/feature-dev-agentic-system.md`](../roadmap/feature-dev-agentic-system.md). Feature-dev lands code; this audit system hardens it — they compose into a loop.
+
+
 ---
 
 ## 1. Design principles (the lessons that shape the architecture)