[Phase 2] GitHub Action for RytScan CI gate

[ToryMic]

Description

Ship a reusable GitHub Action so Wave contributors can add RytScan to contract repos in one workflow step.

Tasks

• Create action.yml with inputs: path, fail-on, rules
• Cache Rust toolchain + RytScan binary
• Annotate PR with finding summary comment
• Add example workflow to README

Acceptance Criteria

• Action runs on PR and fails when high-severity findings exist
• Works on monorepos with contracts/ subdirectory

[YfengJ]

Hi maintainers, I would like to apply for this issue if it is still available.

If assigned, I can add the reusable action.yml with path/fail-on/rules inputs, wire conservative Rust/RytScan caching, add a PR summary comment path, and document a monorepo contracts/ example workflow in the README.

I will wait for maintainer or Stellar Wave assignment/confirmation before cloning, implementing, or opening a PR.