feat: add linux/arm64 release build (#27)

[antoinetoussaint-byte]

Closes #27.

Summary

• Adds a linux-arm64 build stanza to .goreleaser.yaml so releases publish service-rust_<version>_linux_arm64.tar.gz, unblocking the agent on arm64 Linux targets (companion images, CI runners, K8s nodes).
• Uses the aarch64-linux-gnu cross-cgo toolchain already shipped in the goreleaser-cross release image, so no release-workflow change is needed. CGO stays enabled to keep the tree-sitter grammar linked, matching the other stanzas.
• The existing archives name_template already emits {os}_{arch}, so the new asset matches core's downloader contract with no archive change.

Test plan

• goreleaser check passes.
• After merge, cut a release and confirm the asset exists:

  gh release view v<version> --repo codefly-dev/service-rust --json assets \
    --jq '[.assets[].name | select(contains("linux_arm64"))]'