Add local CLI preview and prepare v2.1.0 release

[SLP-DEV1]

Summary

This PR adds a practical local-only CLI foundation and prepares the repository for the first GitHub release.

CLI preview

• process files or directories locally with Sharp
• recursive folder scanning
• WebP, JPG and PNG output
• quality and max-dimension controls
• keep-original-if-larger safety by default
• configurable JPG alpha background
• JSON output for automation/CI
• explicit local-only behavior with no reSmush.it calls

CI

• syntax-check the CLI
• generate a real PNG fixture with Sharp
• run the CLI against it
• assert that a WebP result was produced

Discovery/docs

• add a workflow-focused comparison with Squoosh and TinyPNG/TinyJPG
• avoid synthetic compression benchmark claims
• document CLI examples, options, privacy model and current parity limits

Release

• finalize 2.1.0 release notes
• add a guarded release/v* workflow that validates package version, runs the full validation suite, creates a reproducible source ZIP + SHA256 and publishes the GitHub release

Validation

CI should run npm ci, server syntax, CLI syntax and image smoke-test, strict TypeScript checks, and the production build before merge.