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>

operation/repo/tree_test.go

@@ -44,8 +44,10 @@ func TestSlimTreeNil(t *testing.T) {
 }
 
 func TestGetRepoTreeToolRequired(t *testing.T) {
+	inputSchema := GetRepoTreeTool.InputSchema.(map[string]any)
+	required, _ := inputSchema["required"].([]string)
 	for _, field := range []string{"owner", "repo", "tree_sha"} {
-		if !slices.Contains(GetRepoTreeTool.InputSchema.Required, field) {
+		if !slices.Contains(required, field) {
 			t.Errorf("expected %q to be required", field)
 		}
 	}