v1.2.12

build: switch to Debian base + upload binary to GitHub Release

- Builder/runtime: Alpine (musl) → Debian bookworm-slim (glibc)
  Sprites run Ubuntu 25.04 (glibc); musl binaries are incompatible.
- build-push.yml: after push, pull image and extract the binary,
  upload as opencode-linux-x64 GitHub Release asset.
  Sprites bootstrap by downloading this binary via Services API cmd.
- permissions: contents read → write (GitHub Release upload)