Add go-llm v2: redesigned API for simpler LLM abstraction

v2 is a new Go module (v2/) with a dramatically simpler API:
- Unified Message type (no more Input marker interface)
- Define[T] for ergonomic tool creation with standard context.Context
- Chat session with automatic tool-call loop (agent loop)
- Streaming via pull-based StreamReader
- MCP one-call connect (MCPStdioServer, MCPHTTPServer, MCPSSEServer)
- Middleware support (logging, retry, timeout, usage tracking)
- Decoupled JSON Schema (map[string]any, no provider coupling)
- Sample tools: WebSearch, Browser, Exec, ReadFile, WriteFile, HTTP
- Providers: OpenAI, Anthropic, Google (all with streaming)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/message.go

@@ -0,0 +1,73 @@
+package llm
+
+// Role represents who authored a message.
+type Role string
+
+const (
+	RoleSystem    Role = "system"
+	RoleUser      Role = "user"
+	RoleAssistant Role = "assistant"
+	RoleTool      Role = "tool"
+)
+
+// Image represents an image attachment.
+type Image struct {
+	// Provide exactly one of URL or Base64.
+	URL         string // HTTP(S) URL
+	Base64      string // Raw base64-encoded data
+	ContentType string // MIME type (e.g., "image/png"), required for Base64
+}
+
+// Content represents message content with optional text and images.
+type Content struct {
+	Text   string
+	Images []Image
+}
+
+// ToolCall represents a tool invocation requested by the assistant.
+type ToolCall struct {
+	ID        string
+	Name      string
+	Arguments string // raw JSON
+}
+
+// Message represents a single message in a conversation.
+type Message struct {
+	Role    Role
+	Content Content
+
+	// ToolCallID is set when Role == RoleTool, identifying which tool call this responds to.
+	ToolCallID string
+
+	// ToolCalls is set when the assistant requests tool invocations.
+	ToolCalls []ToolCall
+}
+
+// UserMessage creates a user message with text content.
+func UserMessage(text string) Message {
+	return Message{Role: RoleUser, Content: Content{Text: text}}
+}
+
+// UserMessageWithImages creates a user message with text and images.
+func UserMessageWithImages(text string, images ...Image) Message {
+	return Message{Role: RoleUser, Content: Content{Text: text, Images: images}}
+}
+
+// SystemMessage creates a system prompt message.
+func SystemMessage(text string) Message {
+	return Message{Role: RoleSystem, Content: Content{Text: text}}
+}
+
+// AssistantMessage creates an assistant message with text content.
+func AssistantMessage(text string) Message {
+	return Message{Role: RoleAssistant, Content: Content{Text: text}}
+}
+
+// ToolResultMessage creates a tool result message.
+func ToolResultMessage(toolCallID string, result string) Message {
+	return Message{
+		Role:       RoleTool,
+		Content:    Content{Text: result},
+		ToolCallID: toolCallID,
+	}
+}