v0.2.0 - Metal GPU support (macOS) + nanokimi pretrained model

Adds zero-dependency Metal GPU support on macOS (--gpu, weights cached on device, tuned for large matvecs), plus a leaner README. Includes nanokimi-0.2b.bin + vocab_nano.json: the pretrained 0.2B model (TinyStories, overnight CPU training, no GPU). Download both files into the repo root, then: cargo build --release && ./target/release/microkimi run "Once upon a time" --model nanokimi-0.2b.bin --max-new 12 --raw