feat: add linux/arm64 release build (#72)

[antoinetoussaint-byte]

Closes#72.

Summary

• Adds a linux-arm64 build stanza to .goreleaser.yaml so releases publish service-python_<version>_linux_arm64.tar.gz, which core's downloader already resolves by host arch — arm64 Linux targets 404 today only because the asset was never built.
• Uses the aarch64 cross-cgo toolchain (aarch64-linux-gnu-gcc/g++) already provided by the goreleaser-cross release image, so no release-workflow change is needed. CGO is required because the agent links its own tree-sitter grammar.
• The archivesname_template emits {os}_{arch} already, so the new asset matches the downloader contract with no archive change.

Test plan

• goreleaser check validates the config.
• After merge, cut a new release and verify the asset exists:

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