docs(roadmap): add phase-orchestration + phase-prompts (forward plan & prompts)

Mirror the audit system's structure for the forward pipeline:
- phase-orchestration.md ← finding-orchestration.md: the roadmap DAG as a status
  board (phase × type port/research × status × prerequisites × role/model × chain
  × effort), the ready-set, the recommended waves (0 quick wins / A Phase 12 /
  B genus-g spine), and the per-item gates (spike go/no-go, validation battery,
  review).
- phase-prompts.md ← session-prompts.md: ready-to-paste blocks for the ready-set
  (9g.1, 9h, 9d.3, Phase 12 two-step, 9b-analytic) plus reusable research-spike
  and math-review gate prompts, and the DAG-gated Chain B / G0-blocked notes.
Cross-linked from feature-dev-agentic-system.md.

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

doc/roadmap/feature-dev-agentic-system.md

@@ -9,6 +9,10 @@ finishing the **Java port**, and extending into the **research questions** in
 The two systems **compose into a loop**: feature-dev lands new code → the audit
 system audits & hardens it → findings flow back. Build forward, audit back.
 
+> **Companion files (this system's plan + prompts, mirroring the audit system):**
+> - [`phase-orchestration.md`](phase-orchestration.md) — the DAG/status board (← `finding-orchestration.md`)
+> - [`phase-prompts.md`](phase-prompts.md) — ready-to-paste phase prompts (← `session-prompts.md`)
+
 ---
 
 ## 1. Why this needs a different shape than the audit system