Add ADP support for agent-to-agent communication

## Goal Add ADP support so Elephant Agent instances and sub-agents can communicate through a clear agent-to-agent protocol. ## Scope - Protocol shape for agent identity, capabilities, messages, tasks, and replies - Transport assumptions for local and remote agents - Permission and trust boundaries between agents - Integration points for sub-agent orchestration and future multi-agent workflows ## Acceptance Criteria - ADP requirements and message model are documented. - A minimal protocol interface exists for agent discovery and message exchange. - The implementation can support at least one local agent-to-agent communication path. - Security boundaries are explicit: identity, authorization, and data exposure are covered. - Tests cover basic handshake, message delivery, failure handling, and unsupported capability cases.