Migrate CI from Cirrus CI to GitHub Actions

[fkorotkov-oai]

What changed

• replace Cirrus CI with GitHub Actions for tests, linting, architecture builds, and documentation deployment
• move tag and dry-run releases to GoReleaser Pro
• remove Sentry release side effects
• update the Orchard deployment guide to remove Fury package instructions and use ghcr.io/openai/orchard

Release setup

Before the next release, configure AC_PASSWORD, MACOS_CERTIFICATE, GORELEASER_KEY, and GH_PAT as repository secrets. GH_PAT needs write access to cirruslabs/homebrew-cli.

Validation

• actionlint
• goreleaser check --soft with GoReleaser Pro 2.16.0
• swift build

swift test could not run with the locally active Command Line Tools because XCTest is unavailable there; the migrated test job runs on the Tahoe macOS runner.