Lint GitHub Actions workflows with actionlint

[mattgodbolt]

Part of the org-wide actionlint rollout (see compiler-explorer/compiler-workflows#68, compiler-explorer/infra#2226, compiler-explorer/compiler-explorer#8912). Checks workflow files for schema errors, expression-context mistakes, unknown runner labels, and shellcheck issues (errors+warnings; info/style gated in .github/actionlint.yaml) on any push/PR touching them.

Existing workflows already passed cleanly.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an actionlint-based CI check to validate GitHub Actions workflows in this repository, aligning with an org-wide rollout and using a shared .github/actionlint.yaml configuration to tune rule behavior (notably ShellCheck noise).

Changes:

• Introduces a new workflow to run actionlint on workflow/config changes.
• Adds an actionlint configuration file defining known self-hosted runner labels and ShellCheck ignore patterns for info/style.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/lint-actions.yml	New CI workflow that runs actionlint when workflow/config files change
.github/actionlint.yaml	Adds actionlint configuration (runner labels + ShellCheck ignore tuning)

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[mattgodbolt]

Closing per review: we're keeping the actionlint rollout to the core repos (compiler-explorer, infra, compiler-workflows) rather than adding CI-only lint machinery to quiet single-workflow repos — all our other linters run as pre-commit/local hooks, and this repo's workflows were already clean anyway.