feat(acp): agent_delegate — synchronous agent-to-agent delegation

[DorianZheng]

Summary

• Adds agent_delegate(target_group_jid, prompt, timeout_ms?) action to AcpOutboundClient
• Unlike acp_prompt, this action is synchronous — it awaits the full sub-agent response before returning
• Each call creates an isolated session (no shared caller context)
• Timeout default: 5 minutes, configurable per call via timeout_ms
• Cancels peer session on timeout (best-effort)

Test plan

• Unit tests: success, peer not found, timeout
• E2E tests: success with test-peer, forced timeout
• npm test or bun test passes
• PR opened on boxlite-ai/agentlite