Add Agent Client Protocol v1 support to ds4-agent

[omercelik]

Summary

This adds Agent Client Protocol v1 support to ds4-agent using ACP standard
stdio transport.

The implementation keeps --acp as a separate mode so the existing TUI and
--non-interactive paths keep their current behavior. In ACP mode, stdout is
reserved for protocol messages and diagnostics stay on stderr or trace output.
It covers all non-optional ACP v1 agent features for the standard stdio
transport, and only advertises optional capabilities backed by DS4 behavior.

Supported pieces include:

• initialize
• session/new, session/prompt, session/cancel
• session/list, session/load, session/resume, session/close, session/delete
• session/set_config_option for DS4 browser permission behavior
• session/update notifications for user and assistant text, tool calls, tool results, and config updates
• text and resource prompt blocks, with image and audio rejected because DS4 does not advertise those capabilities
• stdio MCP server configs, MCP tool listing, and MCP tool calls
• ACP permission requests for visible browser startup

ACP reference docs:

• https://agentclientprotocol.com/protocol/v1/overview
• https://agentclientprotocol.com/protocol/v1/transports
• https://agentclientprotocol.com/protocol/v1/schema

Notes

The advertised capabilities are limited to behavior this patch implements. It
does not advertise auth, image/audio support, Streamable HTTP, HTTP MCP, or SSE
MCP.

MCP subprocess writes handle closed pipes without changing the process-wide
SIGPIPE handler, so normal agent modes keep their existing signal behavior.

Testing

• make ds4-agent ds4_test
• ./ds4_test --acp
• ./ds4_test --server
• DS4_TEST_MODEL=<local gguf> make test
• Manual ACP stdio smoke with a local GGUF: initialize, session/new, session/prompt, session/cancel, and session/close; verified JSON-RPC-only stdout and stopReason: "cancelled" for the cancelled prompt.