feat: brain-engine + brain-ui + docs — template full stack standalone

- brain-engine: server, embed, search, RAG, MCP, start.sh (standalone)
- brain-ui: source React complète, build.sh, DocsView avec tier colors
- docs: 14 pages guides humains (getting-started, architecture, sessions, workflows, agents, vues tier)
- brain-compose.yml v0.9.0: tier featured ajouté, sessions/agents par tier, coach_level, API key schema
- DISTRIBUTION_CHECKLIST v1.2: brain-engine + brain-ui + docs dans la checklist

brain-ui/src/components/TierGate.tsx

@@ -0,0 +1,21 @@
+import type { ReactNode } from 'react'
+
+interface TierGateProps {
+  feature: string
+  hasFeature: (f: string) => boolean
+  fallback?: ReactNode
+  children: ReactNode
+}
+
+export default function TierGate({ feature, hasFeature, fallback, children }: TierGateProps) {
+  if (!hasFeature(feature)) {
+    return fallback ? <>{fallback}</> : (
+      <div className="flex flex-col items-center justify-center h-full" style={{ color: '#4b5563' }}>
+        <div className="text-3xl mb-3">🔒</div>
+        <div className="text-sm font-medium">Fonctionnalité non disponible</div>
+        <div className="text-xs mt-1 font-mono" style={{ color: '#374151' }}>{feature} — tier insuffisant</div>
+      </div>
+    )
+  }
+  return <>{children}</>
+}