feat: upgrade to Node 24 and modernize build toolchain

[rathpc]

Summary

Upgrade cli-validation to Node 24, esbuild bundling (replacing @vercel/ncc), ESLint 10, real unit tests, standardized semantic-release with floating major/minor tags, and a CLAUDE.md.

Also fixes two latent upstream issues:

1. Aligns index.js with the explicit-status interface advertised in action.yml. The previous feat: explicit status commit (Nov 2025) updated action.yml and the workflow but never updated index.js or rebuilt dist/. The action has been silently broken since then — action.yml advertises a status input, but the bundled action ignores it and tries to poll the GitHub Actions API for a step-name that action.yml doesn't expose. This PR rewrites index.js to match action.yml: takes status/type/flags from inputs, falls back to YONTRACK_* env vars from a previous cli-config step, and never polls the API.

2. Closes #3 (Add a README). Adds a README.md documenting all inputs, several usage examples, and specifically the if: ${{ !cancelled() }} pattern for unconditional validations the issue called out.

What changed

• Runtime: engines.node set to >=24; action.yml runtime is node24; CI workflow installs Node 24.
• Bundling: replaced @vercel/ncc with esbuild + esbuild-plugin-license (driven by build.js). dist/index.js produced by esbuild; dist/licenses.txt carries third-party attributions.
• Source shape: index.js rewritten in the dependency-injection style of cli-build/cli-setup. A top-level IIFE loads pure-ESM @actions/* packages via dynamic ESM import (namespace, not .default) and calls runAction({core, exec, github}). Source dropped from 184 lines to 71 — the polling machinery is gone.
• Behavior change (the explicit-status migration): removed the step-name/polling logic; the action now reads status/type/flags from inputs as action.yml describes. Project/branch/build resolve from input → YONTRACK_* env var → context fallback (with build requiring at least one of input/env).
• Tests: new index.test.js with 17 unit tests covering required inputs, env-var fallbacks, status pass-through, validation data (type+flags), and the invocation contract. Tests run in <200ms without secrets or network.
• ESLint: eslint 8.x → 10.3.0 with new flat config. .eslintrc.json and .eslintignore removed.
• Release process: new .releaserc matches the standardized template across the repo family. Floating-tag step force-updates vX and vX.Y after each release.
• Docs: new README.md (closes Add a README #3); new CLAUDE.md.
• Workflow: old build.yml replaced by ci.yml with the standardized pipeline.

Why this is breaking

Two reasons:

1. engines.node >= 24 and runs.using: node24 mean self-hosted runners on older Node will fail. GitHub-hosted runners support Node 24 natively.
2. The index.js rewrite removes the (undocumented in action.yml) step-name input the old implementation relied on. The intended interface — status as input — has been the documented contract since the maintainer's feat: explicit status commit; this PR makes the implementation match.

Test plan

• CI passes on this PR (lint, build, tests; self-test against the action is gated to nemerosa + vars.ONTRACK_URL != '' so it skips on fork PRs)
• After merge, semantic-release publishes v3.0.0
• Floating tags v3 and v3.0 exist; existing v2 tags untouched
• CHANGELOG.md is populated by the release commit
• dist/licenses.txt is regenerated alongside the bundle
• Issue Add a README #3 auto-closes when this PR merges

Squash-merge commit message

When merging, please use this exact commit body:

feat: upgrade to Node 24 and modernize build toolchain

- Replace @vercel/ncc with esbuild + esbuild-plugin-license
- Rewrite index.js to match action.yml's explicit-status interface
- Drop the step-name/API-polling implementation (action.yml never advertised it)
- Convert to runAction({core,exec,github}) DI pattern with namespace ESM imports
- Bump @actions/* and semantic-release to latest stable
- Migrate to ESLint 10 flat config
- Add 17 unit tests with mocked GitHub Actions deps
- Standardize release rules (patch on every commit type)
- Add floating major/minor tag step in CI
- Add CLAUDE.md and README.md (closes #3)

BREAKING CHANGE: Now requires Node 24 runtime. Self-hosted runners
must have Node 24 available; GitHub-hosted runners support this
natively. The implementation now matches action.yml's explicit-status
interface — the previous undocumented step-name/polling behavior is
gone. Consumers using @v2 stay on the old code unless they bump to @v3.

Closes #3

[rathpc]

@dcoraboeuf here is another in the batch for your review, thanks!

[dcoraboeuf]

Available in v3.0.0

Thanks for your contribution @rathpc !