Update HalfCheetah-v5 bundle

Update halfcheetah-v5 bundle: context 24->32, return 1768±2624 -> 2680±2059 (50 ep, seed=0)

Add tagline, model details (decoder), training data, and ctx column

Update bundle return table (re-measured 5ep seed=0 CPU; reflects new ant-v5 & humanoid-v5 bundles)

Sync README with repo: drop device=cpu, drop ?nw, add return numbers

Add model card