Cerebro memory enhancement layer for dashboard and gateway

[yacosta738]

Context

Follow-up from #277 (session-memory-visibility, Phase 2).

Goal

When Cerebro MCP is configured, proxy Cerebro memory tools through the gateway and enhance the dashboard with Cerebro-specific memory insights (semantic search, memory graph, ontology rules).

Scope

• Add gateway proxy endpoints for Cerebro MCP memory tools
• Enhance dashboard memory browser with Cerebro semantic search
• Add Cerebro memory insights panel (relationships, ontology)
• Feature-detect Cerebro availability and degrade gracefully

Acceptance Criteria

• Gateway proxies Cerebro MCP tools when configured
• Dashboard shows Cerebro-enhanced memory views when available
• Non-Cerebro deployments continue working unchanged

[linear]

DALLAY-206