Add support for parallel steps (background, wait, wait-all, cancel, parallel)

[devantler]

Summary

GitHub released parallel steps on 2026-06-25, adding five new step keys. actionlint currently rejects any workflow that uses them — e.g. unexpected key "background" for step ..., and step must run script with "run" section or run action with "uses" section for wait/wait-all/cancel/parallel steps — which blocks adoption. This teaches the parser about them.

Closes #693

What the feature looks like

steps:
  - name: Start server
    id: server
    run: npm start
    background: true        # run asynchronously, continue immediately
  - wait: server            # wait for one background step (or `wait: [a, b]`)
  - wait-all:               # wait for all preceding background steps
  - cancel: server          # cancel a single background step
  - parallel:               # run a group of steps in parallel
      - run: ./a.sh
      - run: ./b.sh

Changes

Modeled after the spec in workflow-syntax.md:

• background (boolean) is parsed as a new optional *Bool field on a step (run or action step), mirroring continue-on-error.
• wait/wait-all, cancel and parallel are modeled as new step execution kinds (ExecWait, ExecCancel, ExecParallel) alongside ExecRun/ExecAction, so they no longer trip the "step must run …" check.
  • wait accepts a single step ID or a list (wait: [a, b]); cancel targets a single step ID (a list is reported as a type error, per the spec).
• parallel parses its grouped steps recursively (reusing parseSteps), and the visitor in pass.go recurses into them, so every rule still lints the nested steps.
• Unknown keys on the new step kinds are reported the same way as for run/action steps (e.g. unexpected key "shell" for step to wait for background steps).

Context checking — intentionally deferred (with reason)

actionlint generates its context-availability table (availability.go) from GitHub's contexts.md, and that file does not yet list background/wait/cancel. So there is no published context-availability to validate ${{ }} expressions in those fields against, and adding guessed rules now would risk false positives. Expressions are still parsed (so e.g. background: ${{ ... }} is accepted), and name/if on the new step kinds are checked as before. Once these keys are added to contexts.md, regenerating availability.go plus a small follow-up will cover it.

Test plan

• New fixtures: testdata/ok/parallel_steps.yaml (valid usage of all five keys, incl. an expression-valued background) and testdata/err/parallel_steps.yaml + .out (unexpected keys on wait/cancel/parallel steps; cancel with a list; and an invalid step nested in parallel, which proves the recursion lints it).
• Updated the existing golden fixtures whose step-key lists now include background.
• go test ./... is green (1881 tests); gofmt/go vet/staticcheck clean.

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🤖 Generated with Claude Code

[devantler]

It should be good to merge now. Just verified it against the spec one final time :-)

Thanks for flagging critical oversights @nikosavola!