feat: implement client delete command

[NathaelB]

Implement ferriskey client delete <CLIENT_ID> to delete a client.

Acceptance criteria:

• Resolves UUID from clientId via GET /realms/{realm}/clients?clientId={id} (same pattern as client get)
• Calls DELETE /realms/{realm}/clients/{uuid}
• --realm optional (falls back to context default)
• Confirms deletion in output

Part of #11

[NathaelB]

#15 merged