Implement MCP agent registration model

[NitayRabi]

Summary

• implement the MCP agent registration model across protocol, server, persistence, and web UI
• add MCP control-plane endpoints for register-self, ensure-node, status, and delegate
• represent agents as logical identities with orchestrator and controlled roles, optional node binding, and endpoint metadata
• render orchestrator-only agents in topology and restrict session launch / trigger-rule flows to controllable targets
• add migration coverage and tests for MCP registration, delegation, topology rendering, and role-aware session behavior

Testing

• pnpm test
• pnpm typecheck