Migrate CI from Cirrus CI to GitHub Actions

[fkorotkov-oai]

What changed

• replace Cirrus CI lint, Linux, macOS, and release jobs with GitHub Actions
• keep GoReleaser binary releases and multi-architecture Docker image publishing
• remove nfpm/Fury publishing and obsolete system-package assets
• build the container directly with Go instead of installing GoReleaser Pro in the Dockerfile
• publish the image as ghcr.io/openai/orchard and source Vetu from openai/vetu in Linux CI

Release setup

Before the next release, configure GORELEASER_KEY, GH_PAT, and the existing macOS signing/notarization values as repository secrets. GH_PAT needs write access to cirruslabs/homebrew-cli.

Validation

• actionlint
• goreleaser check --soft with GoReleaser Pro 2.16.0
• go test ./... passed outside hardware-dependent integration packages

The local hardware integration tests could not register an Orchard worker because ioreg is unavailable in this execution environment; the migrated macOS job runs them on the Tahoe runner.