Franklin Sirmans occupies a conspicuously neat space in a complex of glass-front offices at the Los Angeles County Museum of Art. The new head of LACMA’s contemporary art department arrived in January with plenty of ideas, but it takes time to pile up the mountains of books and files that overwhelm many of his colleagues.  Around the corner, Christine Y. Kim has settled in, but just barely. She joined the museum’s staff last September as associate curator of contemporary art. And down the hall, another notably uncluttered office belongs to Britt Salvesen, who came aboard in October as chief of two departments: photography, and prints and drawings. LACMA’s newest curators have landed in prestigious positions at a high-profile institution where their work will be closely watched.