feat(export): add -Prefix parameter to Export-InfisicalSecrets

Adds an optional [string] -Prefix parameter that prepends the supplied
string to every emitted variable name, regardless of -Format
(Json/Yaml/Xml/Env/EnvironmentVariables). When omitted or empty the
exporter buffer is forwarded unchanged (no-op).

Implementation clones each InfisicalSecret with SecretName = Prefix +
SecretName so the caller's pipeline objects are never mutated; the
SecureString and Tags/SecretMetadata array references are shared
(read-only usage downstream).

Also updates the cmdlet help XML description + adds a -Prefix example,
and reflects the new parameter in docs/DesignSpec.md.

docs/DesignSpec.md

@@ -1213,7 +1213,8 @@ Export-InfisicalSecrets `
     [-Path <FileInfo>] `
     [-Scope <Process|User|Machine>] `
     [-Force] `
-    [-Encoding <UTF8|UTF8Bom|Unicode>]
+    [-Encoding <UTF8|UTF8Bom|Unicode>] `
+    [-Prefix <string>]
 ```
 
 ## Parameter Rules