Modernize to node24/ESM, harden defaults, add tests and CI

[rodbegbie]

Summary

• Bumps the action runtime to node24 (GA on GitHub Actions since Oct 2025) and converts source from CommonJS to ESM.
• Bumps uuid (3 → 14), @actions/core (1 → 3); swaps deprecated @zeit/ncc for @vercel/ncc; standardises on npm (drops yarn.lock).
• Hardening from review:
  • Default UUID is now v4 (random) instead of v1 (timestamp-based) — more honest default for a "give me a UUID" action.
  • namespace input is validated with uuid.validate(); bad input produces a clear setFailed message instead of a raw TypeError.
  • Error handler narrows unknown properly: error instanceof Error ? error.message : String(error).
  • Runtime pinned via engines.node and .nvmrc.
• Adds:
  • test/run.test.js — 4 integration tests using node:test that spawn the action with controlled INPUT_* and GITHUB_OUTPUT env vars.
  • .github/workflows/ci.yml — runs the tests and fails if dist/ is out of sync with source.

Breaking change

Default UUID flavour changes from v1 → v4 when no name input is supplied. Consumers relying on the timestamp-ordered v1 default will need to set name/namespace explicitly to get v5, or pin to a previous tag.

Test plan

• CI green on the new workflow (tests + dist-sync check)
• Manually invoke the action in a downstream workflow with no inputs → confirm a v4 UUID is emitted
• Manually invoke with name: hello → confirm a deterministic v5 UUID
• Manually invoke with name: hello, namespace: not-a-uuid → confirm clean failure with the validation message

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