scripts : add release.sh for release preparation (#27497)

Similar to ggml/scripts/release.sh: validates repo state, creates a
release candidate branch (llama-rc-vX.Y.Z), bumps LLAMA_VERSION_* in
CMakeLists.txt and commits the version bump.

Usage: ./scripts/release.sh [major|minor|patch] [--dry-run]

Assisted-by: pi:llama.cpp/Qwen3.8-27B