Add model card: what this is, and which variants are valid

consolidate: drop the duplicated root copy

merged checkpoint ({'pair_id': 'codegen-mono-multi', 'parent_a': 'Salesforce/codegen-350M-mono', 'parent_b': 'Salesforce/codegen-350M-multi', 'ceiling': 'Salesforce/codegen-350M-nl', 'operator': 'average', 'alignment': 'naive', 'align_method': 'permutation', 'regime': 'shared_base', 'eval_langs': 'wikitext', 'nll_merge': 6.3817, 'nll_floor': 6.2402, 'param_coverage': 1.0, 'MS': -0.0891})