feat(acp): add opt-in ACP v2 runtime foundation

[danielkov]

Summary

• keeps the existing ACP v1 API and default behavior
• adds opt-in protocol-v2 support under agentkit_acp::v2
• implements v2 initialize, new, list, resume, prompt, cancel, and close
• accepts prompts immediately and reports running/idle lifecycle updates asynchronously
• provides stable message IDs and visible tool-call content
• releases the non-breaking foundation as agentkit-acp 0.10.8

Compatibility

• root exports remain ACP v1
• ACP v2 is opt-in through the protocol-v2 feature
• dependencies use the released agent-client-protocol = 2.0.0
• no unstable schema or fork dependency is included

Follow-up

• Session injection RFD: docs(rfd): mid-turn input via session/inject (queue and steer) agentclientprotocol/agent-client-protocol#1261
• Stacked AgentKit implementation: feat(acp): add unstable v2 session injection #13