Add CI: build layer and smoke-test it in the Lambda runtime image

[zoellner]

Adds a GitHub Actions workflow (free on public repos with standard runners) and removes eslint.

Workflow Test layer

• syntax-check: npm test (node --check on the repo's JS files; nothing to install)
• build-and-test:
  • builds the layer with sam build --use-container (same public.ecr.aws/sam/build-nodejs24.x image as the documented path), with higher build parallelism for the 4 vCPU / 16 GB runner
  • uploads the built layer as an artifact (7 day retention)
  • mounts the layer at /opt in public.ecr.aws/lambda/nodejs:24 (no toolchain/-devel packages, so missing shared libs fail) and runs test/layer.test.js:
    • dynamic loader resolves every shared library of the compiled sharp addon
    • sharp and libvips versions match layer/Makefile; no prebuilt @img/sharp-* bundled
    • HEIF/WebP/JPEG/PNG input+output support reported
    • decodes examples/src/test-input.heic (3024x4032, hevc)
    • resizes to 200px WebP
    • encodes + decodes HEIF with HEVC (x265) and AV1 (libaom); the HEVC case passes tune: 'ssim' because sharp 0.35's default tune: 'auto' is rejected by libheif's x265 plugin (heif: Invalid parameter value (5.2006))
  • invokes the example function through the Lambda Runtime Interface Emulator and checks for a 200 response with a WebP body

Runs on PRs, pushes to main, and manual dispatch; superseded PR runs are cancelled.

Remove eslint

eslint was the only dependency of the root package and the source of all 10 open Dependabot alerts (transitive deps in package-lock.json). Removed the devDependency, eslint.config.js and the lint scripts; npm test now runs node --check, also used by buildspec.yml in place of npm run lint. The lockfile has no dependencies left and npm audit is clean. Supersedes #45–#49.