Implement OpenAI-to-Anthropic proxy with streaming support

- Add request/response converters (OpenAI <-> Anthropic formats)
- Implement SSE streaming conversion (Anthropic events -> OpenAI SSE)
- Add /v1/models endpoint with Claude model list
- Add /v1/chat/completions endpoint with streaming and non-streaming support
- Fix context key type matching bug (sessionIDKey)
- Configurable upstream URL via config.yaml
- Mimic claude-code CLI headers for upstream requests

main.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+
+	"github.com/google/uuid"
+	"gopkg.in/yaml.v3"
+)
+
+func main() {
+	// Load config.yaml
+	data, err := os.ReadFile("config.yaml")
+	if err != nil {
+		log.Fatalf("Failed to read config.yaml: %v", err)
+	}
+	cfg := &Config{}
+	if err := yaml.Unmarshal(data, cfg); err != nil {
+		log.Fatalf("Failed to parse config.yaml: %v", err)
+	}
+	config = cfg
+
+	// Generate session ID (persist across requests)
+	sessionID := uuid.New().String()
+
+	// Register routes
+	http.HandleFunc("/v1/chat/completions", func(w http.ResponseWriter, r *http.Request) {
+		r = r.WithContext(contextWithSessionIDInContext(r.Context(), sessionID))
+		handleChatCompletions(w, r)
+	})
+	http.HandleFunc("/v1/models", handleModels)
+	http.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprint(w, "OK")
+	})
+
+	addr := fmt.Sprintf(":%d", config.Port)
+	log.Printf("Starting proxx on %s, upstream: %s", addr, config.UpstreamURL)
+	if err := http.ListenAndServe(addr, nil); err != nil {
+		log.Fatalf("Server failed: %v", err)
+	}
+}
+
+// contextKey is a custom type for context keys
+type contextKey string
+
+const sessionIDKey contextKey = "sessionID"
+
+// contextWithSessionID creates a context with the session ID
+func contextWithSessionID(sessionID string) context.Context {
+	return context.WithValue(nil, sessionIDKey, sessionID)
+}
+
+// contextWithSessionIDInContext creates a new context with session ID in existing context
+func contextWithSessionIDInContext(parent context.Context, sessionID string) context.Context {
+	return context.WithValue(parent, sessionIDKey, sessionID)
+}