feat: upgrade to Node 24 and modernize build toolchain

[rathpc]

Summary

Upgrade cli-install to Node 24, esbuild bundling (replacing @vercel/ncc), ESLint 10, real unit tests, standardized semantic-release with floating major/minor tags, and a new CLAUDE.md. Pins to @nemerosa/ontrack-github-actions-module-install@^2.0.0 (the new major from its just-merged Wave 1 upgrade).

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. Stale ncc artifact (dist/sourcemap-register.js) removed.
• Source shape: index.js refactored to a dependency-injection pattern. A top-level IIFE loads pure-ESM @actions/core via dynamic ESM import (namespace, not .default) and calls runAction({core, client}). @nemerosa/ontrack-github-actions-module-install stays as a top-level require() since its CJS exports are preserved.
• Tests: new index.test.js with 7 unit tests covering input handling (url/token inputs + env-var fallbacks + precedence), parameter pass-through to client.install, and outputs/PATH side effects. Tests run in <200ms without secrets or network.
• Module dep: @nemerosa/ontrack-github-actions-module-install bumped from ^1.2.1 to ^2.0.0.
• @actions/core: 1.x → 3.x (now consumed via dynamic ESM import).
• ESLint: eslint 8.x → 10.3.0 with new flat config. .eslintrc.json and .eslintignore removed.
• Release process: new .releaserc matches the standardized template; floating-tag step force-updates vX and vX.Y after each release. Pre-release "commit dist" step removed — semantic-release's git plugin handles dist/ via assets.
• Workflow: Node 20 → 24, actions/checkout@v4 → @v6. "Test: local configuration" gated on vars.YONTRACK_URL != '' so fork PRs without that var don't fail.
• Docs: new CLAUDE.md.

Why this is breaking

engines.node >= 24 and runs.using: node24 mean self-hosted runners on older Node will fail. GitHub-hosted runners support Node 24 natively.

Test plan

• CI passes on this PR (lint, build, tests; "Test: download" + "Test: getting the version" run on internal PRs and main pushes; "Test: local configuration" only when vars.YONTRACK_URL is set)
• After merge, semantic-release publishes the next version with floating tags

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
- Convert index.js to runAction({core,client}) DI pattern
- Bump @actions/core to v3 with namespace dynamic-import
- Pin @nemerosa/ontrack-github-actions-module-install to ^2.0.0
- Migrate to ESLint 10 flat config
- Add 7 unit tests with mocked deps
- Standardize release rules (patch on every commit type)
- Add floating major/minor tag step in CI
- Add CLAUDE.md for AI-assisted development

BREAKING CHANGE: Now requires Node 24 runtime. Self-hosted runners
must have Node 24 available; GitHub-hosted runners support this
natively. Module-install dep bumped to ^2.0.0.

[rathpc]

@dcoraboeuf here is another in the batch for your review

[dcoraboeuf]

Available in v2.0.0

Thanks for your contribution @rathpc