Add MCP client support to ds4-agent

[aovestdipaperino]

Summary

Adds Model Context Protocol (MCP) client support to ds4-agent, so external stdio MCP servers can extend the agent with new tools without touching C code.

• Config: servers are declared in a .mcp.json file (same shape Claude Code uses: mcpServers: {name: {command, args, env}}), read from the working directory or via the new --mcp-config FILE flag. MCP is fully optional: a missing file is silently fine, a malformed one warns and is skipped, and a server that fails to spawn or handshake is dropped with a warning instead of aborting the agent.
• Transport: each server is a long-lived subprocess speaking newline-delimited JSON-RPC 2.0 on stdin/stdout (the MCP stdio transport), driven by a small self-contained JSON parser/writer added to ds4_agent.c. Tool calls stay synchronous like every other agent tool; requests carry a 30s whole-exchange deadline so a misbehaving server cannot hang a turn.
• Tool exposure: discovered tools are namespaced mcp__<server>__<tool> and rendered into the system prompt in the same function-call JSON shape as the native tools. Dispatch plugs into agent_execute_tool_call, and results flatten to plain text like every other tool.
• Prompt-size control: an optional per-server primaryTools list splits tools into primary (full schema in the prompt) and directory entries (one line each); the model fetches directory schemas on demand with the new native mcp_describe tool. With tokensave's 81 tools this shrinks the MCP prompt block by ~71%.

Testing

• make stays warning-clean; ds4_agent_test grows 13 unit tests covering the JSON round-trip, config parsing (including rejection of __ in server names), DSML-args→JSON-RPC conversion, primary/directory prompt rendering, and mcp_describe resolution.
• Live-checked end to end against a scripted stdio MCP server (spawn, handshake, tools/list, tools/call round trip) and against a real 81-tool server, including the on-demand schema path.

🤖 Generated with Claude Code