feat(config): env-var loading

Add internal/config package with Config struct and Load() function.
Reads GITEA_BASE_URL, GITEA_API_TOKEN, GITEA_MCP_ALLOWED_OWNERS,
GITEA_MCP_ORIGIN_ALLOWLIST, GITEA_MCP_PORT with sensible defaults.
Wire cfg.Port into main.go. TDD: tests written first, then impl.

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

cmd/gitea-mcp/main.go

@@ -4,18 +4,26 @@ import (
 	"log/slog"
 	"net/http"
 	"os"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/config"
 )
 
 func main() {
 	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
 
+	cfg, err := config.Load()
+	if err != nil {
+		logger.Error("load config", "err", err)
+		os.Exit(1)
+	}
+
 	mux := http.NewServeMux()
 	mux.HandleFunc("/healthz", func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusOK)
 		_, _ = w.Write([]byte("ok"))
 	})
 
-	addr := ":8080"
+	addr := ":" + cfg.Port
 	logger.Info("gitea-mcp starting", "addr", addr)
 	if err := http.ListenAndServe(addr, mux); err != nil {
 		logger.Error("server stopped", "err", err)