docs(0.28.0): CHANGELOG entry + README BOM bump and What's New

[michalharakal]

• CHANGELOG: add [0.28.0] - 2026-06-06 (reshape DAG DSL: reshape drops shape; RoPE/groupNorm/upsample have no op; and dag{} lacks builders for supported NN ops (layerNorm/rmsNorm/batchNorm/silu/embedding) #666, concatenate DAG DSL: multi-input concatenate mis-infers the concatenated axis (0.27.0) #667, constants/reductions StableHLO export from the DSL produces no usable module (0.27.0): tracing emits no ops; DAG externalizes constants & keeps reduced dim #663, HloGenerator tracing HloGenerator.generate emits an empty module: it doesn't pushTape onto Execution.tapeStack around forward #668, non-JVM image runtime Update KMP support for data transform APIs #671)
• README: bump skainet-bom 0.27.0 -> 0.28.0; new What's New in 0.28.0; demote 0.27.0 into Recent releases