#992 ci: add aireceipts pr-check (notice-only cost receipt)

Adds a notice-only aireceipts cost-receipt check (pr-check) — one self-contained workflow, no external reusable workflow, no org Actions-policy change. Never fails a build; posts an aireceipts cost receipt only once a branch carries an aireceipts receipt ref, otherwise a quiet no-op. Coexists with any existing check; remove anytime by deleting the file. Part of an org-wide aireceipts dogfood.

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Summary by cubic

Add a notice-only aireceipts PR check to post AI cost receipts on pull requests. It never fails builds and is a no-op unless the branch has an aireceipts receipt ref.

• New Features
  • Adds .github/workflows/aireceipts.yml that runs on pull requests.
  • Executes npx -y aireceipts-cli@latest pr-check with GH_TOKEN.
  • Posts a cost receipt comment when a receipt ref is present; otherwise quiet.
  • Self-contained and removable by deleting the workflow file.

Summary by CodeRabbit

• Chores
  • Added an automated pull request check to help validate changes during review.
  • The new workflow runs on pull request events and posts its results back to the PR.