Add minimal Agent Client Protocol (ACP) stdio mode to ds4-agent

[omercelik]

Summary

This adds a minimal Agent Client Protocol mode to ds4-agent behind --acp. ACP lets editors and other clients drive the agent over stdio using newline delimited JSON-RPC.

The implementation keeps stdout reserved for protocol messages in ACP mode and leaves the existing TUI and --non-interactive paths unchanged.

Scope

• adds --acp
• handles initialize, session/new, session/prompt, and session/cancel
• streams assistant text and tool updates through session/update
• supports baseline text and resource_link prompt blocks
• returns standard JSON-RPC errors for parse, request, method, params, and internal failures

Not included

• session/load or session/list
• modes or config options
• client filesystem or terminal methods
• ACP permission requests

References

• Overview: https://agentclientprotocol.com/protocol/overview
• stdio transport: https://agentclientprotocol.com/protocol/transports
• Prompt turn: https://agentclientprotocol.com/protocol/prompt-turn
• Schema: https://agentclientprotocol.com/protocol/schema

Test environment

• Machine: Apple M4 Max MacBook Pro, 128 GB RAM
• OS: macOS 26.4.1
• Backend: Metal
• Test model: DeepSeek-V4-Flash-IQ2XXS-w2Q2K-AProjQ8-SExpQ8-OutQ8-chat-v2-imatrix.gguf

Tests

• make clean
• make
• make test
• ./ds4_test --server
• ./ds4_test --acp
• make cpu
• git diff --check

[omercelik]

Closing this in favor of #354, which keeps the same ACP stdio entry point but includes the full non-optional ACP v1 agent coverage and supported DS4 optional capabilities.