Rename Go client package from mciasgoclient to mcias

- Update package declaration in client.go
- Update error message strings to reference new package name
- Update test package and imports to use new name
- Update README.md documentation and examples with new package name
- All tests pass

clients/go/client.go

@@ -1,8 +1,8 @@
-// Package mciasgoclient provides a thread-safe Go client for the MCIAS REST API.
+// Package mcias provides a thread-safe Go client for the MCIAS REST API.
 //
 // Security: bearer tokens are stored under a sync.RWMutex and are never written
 // to logs or included in error messages anywhere in this package.
-package mciasgoclient
+package mcias
 
 import (
 	"bytes"
@@ -28,7 +28,7 @@ type MciasError struct {
 }
 
 func (e *MciasError) Error() string {
-	return fmt.Sprintf("mciasgoclient: HTTP %d: %s", e.StatusCode, e.Message)
+	return fmt.Sprintf("mcias: HTTP %d: %s", e.StatusCode, e.Message)
 }
 
 // MciasAuthError is returned for 401 Unauthorized responses.