[WIP] Implement in-process actionlint linting using Go API

[Copilot]

• Replace Docker-based actionlint with in-process actionlint.Linter Go API in pkg/cli/actionlint.go
  • Replace getActionlintVersion to use runtime/debug build info instead of Docker
  • Add displayActionlintErrors([]*actionlint.Error) helper for native error display
  • Replace runActionlintOnFiles to use actionlint.NewLinter + LintFiles
• Remove ActionlintImage constant from pkg/cli/docker_images.go
• Update CheckAndPrepareDockerImages to not require Docker for actionlint
• Update docker_images_test.go tests that reference ActionlintImage
• Add TestDisplayActionlintErrors test in actionlint_test.go
• Run make agent-finish to validate all changes

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