fix(cli): correct article for credential primitive ('a' not 'an')

The article was 'an' for the old 'identity' label but 'credential'
takes 'a'. This fixes help text like "Remove an credential" → "Remove a credential".

Confidence: high
Scope-risk: narrow

Author: jesseturner21

src/cli/primitives/CredentialPrimitive.tsx

@@ -67,7 +67,7 @@ export class CredentialPrimitive extends BasePrimitive<AddCredentialOptions, Rem
   readonly label = 'Credential';
   readonly primitiveSchema = CredentialSchema;
 
-  protected override readonly article: string = 'an';
+  protected override readonly article: string = 'a';
 
   async add(options: AddCredentialOptions): Promise<AddResult<{ credentialName: string }>> {
     try {