Add minimal MCP server helper for tool exposure

[elkraneo]

Use case: apps exposing custom tools to ACP agents currently need to hand-roll an MCP server (HTTP or stdio). I ended up copying a full MCP HTTP bridge (JSON-RPC parsing, OAuth stubs, etc.) just to serve tools/list and tools/call.

Suggestion: ship a tiny MCP server helper in swift-acp, e.g.:

MCPServer.start(
  transport: .http(localhost),
  toolProvider: () async -> [ToolDefinition],
  toolHandler: (name, args) async throws -> CallToolResponse
)

• Or MCPServer.startStdio(...) for CLI targets.

Even a minimal, no-auth implementation would remove a lot of boilerplate and make tool exposure consistent across apps.