refactor!: replace mcp-go with the official MCP Go SDK

- Swap the mark3labs MCP dependency for modelcontextprotocol/go-sdk v1.7.0
- Add a declarative tool definition and JSON Schema builder to the tool package
- Adapt registered tools to the official low-level handler, recovering panics and mapping failures to JSON-RPC errors
- Narrow tool handlers to take a plain argument map instead of an SDK request type
- Rewire stdio and HTTP transports onto the official server, moving Authorization parsing into receiving middleware
- Add a golden contract test that locks the exposed tool schemas, plus SDK integration and helper tests
- Add a test target and run it in the pull request workflow

BREAKING CHANGE: The exported helpers change signature. Tool.RegisterRead and
Tool.RegisterWrite now take tool.ServerTool instead of server.ServerTool, and the
annotation constructors return *mcp.ToolAnnotations from the official SDK. Callers
must build tool definitions with tool.NewDefinition and handlers with the
func(context.Context, map[string]any) signature.

The 30-second SSE heartbeat is removed because the official SDK has no equivalent.
ServerOptions.KeepAlive is deliberately not used as a substitute, since it sends MCP
ping requests and ping is removed in protocol 2026. HTTP stays stateless=false, so
new clients negotiate at most 2025-11-25.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/tool/definition_test.go

@@ -0,0 +1,80 @@
+package tool
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+func TestNewDefinition(t *testing.T) {
+	annotations := &mcp.ToolAnnotations{Title: "Example", ReadOnlyHint: true}
+	definition := NewDefinition(
+		"example",
+		"Example tool",
+		annotations,
+		String("owner", Required(), Description("repository owner"), Enum("one", "two"), Default("one")),
+		Number("page", Required(), Default(1), Minimum(1)),
+		Boolean("draft"),
+		Array("labels", Items(map[string]any{"type": "string"})),
+		Object("inputs", Description("workflow inputs")),
+	)
+
+	if definition.Name != "example" || definition.Description != "Example tool" {
+		t.Fatalf("definition = %#v", definition)
+	}
+	if definition.Annotations != annotations {
+		t.Fatal("NewDefinition did not preserve annotations")
+	}
+
+	want := map[string]any{
+		"type": "object",
+		"properties": map[string]any{
+			"owner": map[string]any{
+				"type":        "string",
+				"description": "repository owner",
+				"enum":        []string{"one", "two"},
+				"default":     "one",
+			},
+			"page": map[string]any{
+				"type":    "number",
+				"default": 1,
+				"minimum": float64(1),
+			},
+			"draft": map[string]any{"type": "boolean"},
+			"labels": map[string]any{
+				"type":  "array",
+				"items": map[string]any{"type": "string"},
+			},
+			"inputs": map[string]any{
+				"type":        "object",
+				"properties":  map[string]any{},
+				"description": "workflow inputs",
+			},
+		},
+		"required": []string{"owner", "page"},
+	}
+	if !reflect.DeepEqual(definition.InputSchema, want) {
+		t.Errorf("InputSchema = %#v, want %#v", definition.InputSchema, want)
+	}
+
+	data, err := json.Marshal(definition)
+	if err != nil {
+		t.Fatalf("json.Marshal() error = %v", err)
+	}
+	if !json.Valid(data) {
+		t.Fatalf("json.Marshal() returned invalid JSON: %s", data)
+	}
+}
+
+func TestNewDefinitionWithoutRequiredProperties(t *testing.T) {
+	definition := NewDefinition("empty", "", nil)
+	schema := definition.InputSchema.(map[string]any)
+	if _, ok := schema["required"]; ok {
+		t.Errorf("InputSchema unexpectedly contains required: %#v", schema)
+	}
+	if got := schema["properties"]; !reflect.DeepEqual(got, map[string]any{}) {
+		t.Errorf("properties = %#v, want empty map", got)
+	}
+}