docs: add State Hub reference page and restructure reference index

New page (docs/state-hub.md) covers:
- Why: the invisible state problem across repos and agents
- What: Derived Data Store, Read Model, Agent Orchestration Layer,
  Cross-Repo Observatory — and what it is NOT
- Derived Data Store principle (ADR-003): fingerprint cache, rebuild
  guarantee, force-refresh
- Repository Orchestrator: session protocol, cross-domain coordination
  via messages + capability routing, Kaizen agents
- Architecture diagram (ASCII), technology choices, data model overview
- Running the hub, design principles, related docs

reference.md: add Architecture & Design section grouping state-hub,
TPSC, GDPR maturity, SCOPE.md, capabilities, and goals docs.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

dashboard/observablehq.config.js

@@ -86,6 +86,7 @@ export default {
         { name: "Ralph Workplan",           path: "/docs/ralph-workplan" },
         { name: "Reference & Context Help", path: "/docs/reference" },
         { name: "Repo Integration",         path: "/docs/repo-integration" },
+        { name: "State Hub",                path: "/docs/state-hub" },
         { name: "Repos",                    path: "/docs/repos" },
         { name: "SBOM",                     path: "/docs/sbom" },
         { name: "SCOPE.md",                 path: "/docs/scope" },