MCP client: spawn servers, capability handshake, tool discovery

[humancto]

Status

MCP client already exists in crates/punch-runtime/src/mcp.rs — fully implemented with:

• Subprocess spawning with stdio transport (JSON-RPC 2.0)
• Tool discovery (tools/list)
• Tool invocation (tools/call)
• Error handling, timeouts, comprehensive tests

What is NOT wired in

• fighter_loop.rs — No calls to McpClient
• tool_executor.rs — No MCP tool execution logic
• Ring — No MCP server lifecycle management
• PunchConfig — No mcp_servers config field
• FighterManifest — No MCP capability gating

Work Required

1. Add mcp_servers: HashMap<String, McpServerConfig> to PunchConfig
2. Integrate McpClient startup/shutdown into Ring lifecycle
3. Register discovered MCP tools in the tool executor
4. Route MCP tool calls from fighters to the correct server
5. Add mcp_allowlist capability to FighterManifest
6. End-to-end integration tests

Context

The hard part (protocol implementation) is done. This is integration work — wiring the existing client into the Ring and fighter loop.