Move non-core root reference docs under docs/reference

docs/reference/PORTAL_MIGRATION.md

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+# Portal Migration — ZeroLagHub
+
+This document tracks the transition from legacy console behavior to the current
+agent-driven, PTY-backed architecture.
+
+---
+
+## Current State (Feb 2026)
+
+- All server consoles are PTY-backed and agent-owned.
+- Frontend streams output via WebSocket (single writer).
+- Game and dev consoles are **route-separated** but share components.
+- Metrics and player presence are sourced server-side (API → agent).
+- No frontend log scraping or client-side metrics access.
+
+---
+
+## Migration Notes
+
+- Pterodactyl-style assumptions (polling logs, iframe dashboards) are explicitly not used.
+- Grafana is reserved for internal/admin use only.
+- Customer-facing metrics are API-mediated summaries.
+- Game-specific context is additive and isolated to game routes.
+
+---
+
+## Remaining Migration Work
+
+- Final UI polish and UX consistency checks
+- Concurrent provisioning validation
+- Formal GA readiness checklist