[codex] Migrate CI to GitHub Actions

[fkorotkov]

What changed

• Replaced Cirrus CI release jobs with GitHub Actions jobs on standard ubuntu-24.04 runners.
• Replaced the existing Cirrus-labeled GitHub runner with the standard GitHub-hosted ubuntu-24.04 runner.
• Added Actions jobs for linting, integration tests, release dry runs, tagged releases, and artifact upload.
• Switched release credentials from Cirrus encrypted values to GitHub Actions secrets (FURY_TOKEN, GORELEASER_KEY) plus the built-in GITHUB_TOKEN.

Dropped bare-metal coverage

No bare-metal-only Cirrus tasks were present in this repository's Cirrus configuration, so no tasks were dropped for this migration.

Validation

• Parsed .github/workflows/main.yml locally with Ruby YAML.
• Ran git diff --check.
• Confirmed no Sentry references remain in the migrated workflow files.

actionlint was not installed locally, so I could not run workflow semantic linting before opening this draft.