examples: add a 'listeverything' client example, and reorganize

Add a client example that lists all features on a stdio server. Since
this is our first client example, organize examples by client / server
-- otherwise it is too difficult to find client examples.

For #33

examples/client/listfeatures/main.go

@@ -0,0 +1,65 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+// The listfeatures command lists all features of a stdio MCP server.
+//
+// Usage: listfeatures <command> [<args>]
+//
+// For example:
+//
+//	listfeatures go run github.com/modelcontextprotocol/go-sdk/examples/server/hello
+//
+// or
+//
+//	listfeatures npx @modelcontextprotocol/server-everything
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"iter"
+	"log"
+	"os"
+	"os/exec"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+func main() {
+	flag.Parse()
+	args := flag.Args()
+	if len(args) == 0 {
+		fmt.Fprintf(os.Stderr, "Usage: listfeatures <command> [<args>]")
+		fmt.Fprintf(os.Stderr, "List all features for a stdio MCP server")
+		fmt.Fprintln(os.Stderr)
+		fmt.Fprintf(os.Stderr, "Example: listfeatures npx @modelcontextprotocol/server-everything")
+		os.Exit(2)
+	}
+
+	ctx := context.Background()
+	cmd := exec.Command(args[0], args[1:]...)
+	client := mcp.NewClient(&mcp.Implementation{Name: "mcp-client", Version: "v1.0.0"}, nil)
+	cs, err := client.Connect(ctx, mcp.NewCommandTransport(cmd))
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer cs.Close()
+
+	printSection("tools", cs.Tools(ctx, nil), func(t *mcp.Tool) string { return t.Name })
+	printSection("resources", cs.Resources(ctx, nil), func(r *mcp.Resource) string { return r.Name })
+	printSection("resource templates", cs.ResourceTemplates(ctx, nil), func(r *mcp.ResourceTemplate) string { return r.Name })
+	printSection("prompts", cs.Prompts(ctx, nil), func(p *mcp.Prompt) string { return p.Name })
+}
+
+func printSection[T any](name string, features iter.Seq2[T, error], featName func(T) string) {
+	fmt.Printf("%s:\n", name)
+	for feat, err := range features {
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("\t%s\n", featName(feat))
+	}
+	fmt.Println()
+}