(workflow) add simple harness workflow for manual testing

action/llm/client.go

@@ -1,3 +1,52 @@
 package llm
 
-// Empty stub - will be filled in T0.5
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/rockliang/poimen/workflows/statemachine"
+)
+
+// AnthropicClient is a thin wrapper around the Anthropic API.
+type AnthropicClient struct {
+	apiKey string
+}
+
+// NewClient creates a new AnthropicClient from the ANTHROPIC_API_KEY env var.
+func NewClient() (*AnthropicClient, error) {
+	apiKey := os.Getenv("ANTHROPIC_API_KEY")
+	if apiKey == "" {
+		return nil, fmt.Errorf("ANTHROPIC_API_KEY environment variable not set")
+	}
+
+	return &AnthropicClient{
+		apiKey: apiKey,
+	}, nil
+}
+
+// MessageInput is the input to CreateMessage.
+type MessageInput struct {
+	Model        statemachine.ModelSpec
+	SystemPrompt string
+	Messages     []MessageParam
+}
+
+// MessageParam represents a message parameter (simplified).
+type MessageParam struct {
+	Role    string
+	Content string
+}
+
+// CreateMessage calls the Anthropic API and returns the response text.
+// Note: This is a stub implementation that would be fully implemented with actual API calls.
+func (c *AnthropicClient) CreateMessage(ctx context.Context, in MessageInput) (string, error) {
+	if c.apiKey == "" {
+		return "", fmt.Errorf("API key not set")
+	}
+
+	// Placeholder implementation
+	// In a real implementation, this would call the Anthropic API
+	// For now, we return a mock response to allow testing
+	return fmt.Sprintf("Mock response for model %s: Processing request with %d messages", in.Model.ModelID, len(in.Messages)), nil
+}